1. Total entry, irreversible loss and recycling rates of glucose were measured in four non-pregnant female Shetland ponies before and after a 24 h fast by the continuous intravenous infusion of a mixture of [U-14C] glucose and [2-3H]glucose.

2. The post-fasting fall in the concentration of glucose and the rise in the concentration of ketones in plasma were not significant.

3. After fasting, the total entry rate fell from 1·44±0·11 (n 4) to 1·19±0·12 mg/min per kg body-weight (P > 0·01) and irreversible loss fell from 1·36±0·10 (n 4) to 1·05±0·10 mg/min per kg body-weight (P > 0·02). Recycling rate did not change significantly.

4. In one pony the intravenous infusion of glucose at rates of 0·52, 1·0 or 1·64 mg/min per kg body-weight had no appreciable effect on endogenous glucose production whereas in a second pony, similarly fed, total entry was depressed by 43% and irreversible loss by 54%.

5. Irreversible loss of glucose in the horse is similar to that of many other species.

1. The regulation of the utilization of dietary tryptophan, nicotinamide and nicotinic acid as precursors of the nicotinamide nucleotides has been studied in groups of rats fed on diets providing only one of these precursors at a time, in amounts adequate to meet their requirements for nucleotide synthesis.

2. The concentration of nicotinamide nucleotides in the liver of rats receiving a high-tryptophan diet was 56% higher than in animals fed on a diet providing a minimum amount of tryptophan, together with nicotinic acid or nicotinamide. The excretion of N1-methyl nicotinamidc was three times higher in the tryptophan-fed animals than in the other two groups.

3. The concentration of quinolinic acid in the liver was significantly higher in animals receiving the high-tryptophan diet than in the other two groups; that of nicotinic acid was highest in those animals receiving the nicotinic-acid-containing diet. The concentration of nicotinamide was highest in the livers of those animals receiving the high-tryptophan diet, and lowest in those receiving the nicotinic-acid-contaning diet.

4. The values of the Michaelis constant Km of nicotinamide deamidase (nicotinamide amidohydrolase. EC 3.5,1,19) and nicotinamide phosphoribosylltransferase (nicotinamide nucleotide: pyrophosphate phosphoribosyltransferase, EC 2.4.2.12) were approximately equal, and approximately one-tenth of the concentration of nicotinamide was in the liver. This suggests that both these enzymes normally function at their maximum rate, and a change in the availability of nicotinamide would not affect the rate of its incorporation into nucleotides.

5. The maximum rate of reaction (Vmax) of nicotinamide deamidase was twice that of nicotinamide phosphoribosyltransferase; this suggests that unless compartmental or other factors are involved, the major route of nicotinamide utilization will be by way of deamidation.

6. The Km of nicotinate phosphoribosyltransferase (nicotinate nucleotide: pyrophosphate phosphoribosyltransferase, EC 2.4.2.11) was less than twice the concentration of nicotinic acid in the liver, so that a change in the availability of nicotinic acid might be expected to lead to a small change in the rate of its utilization.

7. The Km of quinolinate phosphoribosyltransferase (nicotinate nucleotide: pyrophosphate phosphoribosyltransferase (carboxylating) EC 2.4.2.19) was approximately 100 times greater than the concentration of quinolinic acid in the liver, so that any change in the availability of quinolinic acid would be expected to lead to a considerable change in the rate of its utilization. The Vmax of quinolinate phosphoribosyltransferase was relatively low, so that under conditions of high tryptophan flux, some accumulation of quinolinic acid might be expected. This was observed in animals receiving the high-tryptophan diet.

8. It is concluded that it is unlikely that the utilization of quinolinic acid, arising from tryptophan. for the synthesis of nicotinamide nucleotides is regulated, but that control over tissue concentrations of nucleotides is achieved by hydrolysis of NAD to nicotinamide. Incorporation of nicotinamide into nucleotides seems to be strictly limited by the activity of the enzymes involved.

1. The metabolism of acetate, glucose and D(−)-3-hydroxybutyrate was studied in lactating and non lactating sheep in vivo. Special consideration was given to the utilization by hind-limb muscle in both groups of sheep and the uptake of nutrients by the lactating mammary gland was also measured.

2. The entry of acetate into the circulation (mmol/h per kg body-weight) was similar in all experimental animals at a given arterial concentration of acetate. However, normal lactation was associated with a reduced extraction of acetate by muscle and the 'spared' acetate was comparable with that removed by the udder. Feeding lactating ewes a 700 g concentrate/kg ration tended to prevent this redistribution of acetate utilization.

3. The muscle of non-lactating ewes utilized sufficient glucose, when corrected for lactate release, to account for 57% of the oxygen utilization by muscle. In lactation this fell to 32%, largely because of an increased lactate production. D(−)-3-Hydroxybutyrate utilization by muscle accounted for 16–17% of the O2 consumed by the muscle in non-lactating and lactating sheep.

4. Lactating mammary gland metabolism in sheep was similar to published values for dairy cows and goals. Thus the extraction (%) of glucose, O2, acetate and D(−)-3-hydroxybutyrate was 25, 28, 62 and 53 respectively. Blood flow was 529 ml/min per kg udder and the ratio blood flow: milk flow was 475. Glucose used by the udder relative to the whole animal utilization rate may be less in sheep than in cows and goats, but the comparable proportion for acetate is as large or larger than in these species.

1. Digesta passage and retention were measured in heifers, sheep, goats, equines and rabbits of varying body-weights when given timothy (Phleum pratense) hay.

2. Two passage markers were compared, cobalt (III) ethylene diamine tetraacetate (CoEDTA) and chromiummordanted timothy fibre for liquid and solid phase respectively. Both markers were injected into the rumen of the ruminants and into the caecum of the equines andrabbits.

3. In ruminants, two different sets of rate constants (k1 and k2) were derived from a two-pool model for marker passage, using a graphical approach and a computer-based non-linear least-squares curve-fitting technique.

4. Retention times, due to unidirectional flow through the gastrointestinal tract (transit time) and due to pool effects (mean retention time, MRT), were calculated.

5. Curve fitting was only successful for the excretion of liquids in ruminants. The two-pool model was not applicable to the passage of solids.

6. Apparent retention of liquid was always shorter than for solids in all species, except in rabbits. However absorption of CoEDTA was too large in the rabbits to determine liquid retention accurately. Times for first appearance of the two markers were similar within animal groups.

7. MRT values were lowest in the rabbit, intermediate in equines and high in the ruminants. The MRT values (h) of solids and liquid respectively were: large heifers 65, 18; small heifers 48, 20; goats 41, 28: sheep 57, 26: equines 23, 18; rabbits 5·3, not determined.

8. Liquid retention seemed to decrease somewhat with increasing body-weight in the ruminants. Solids retention decreased with decreasing body-weight in the ruminants, but sheep had longer retention times than goats of similar size. Equines exhibited large individual variation in retention of the liquid or solid markers, seemingly unrelated to size. Noeffect of size was seen in the retention of solids in the rabbits.

1. Bipolar electrodes were permanently implanted on the gastric antrum, and on the different portions of the small intestine of each of eleven healthy adult cats receiving one meal daily. All parts of the feline gut exhibited, as in several other species, regular slow waves and alternate periods of quiescence and electrical spiking activity during the recording sessions lasting from 10 to 30 d.

2. Patterns of electrical activity characteristic of this species were identified. Both the amplitude and frequency of the antral slow-wave were related to the presence cf superimposed spike bursts during fasting decrease in the antral slow-wave frequency and increase in the length of the duodenal plateau of slow waves after the daily meal were related to its nature.

3. In fasted state, the electrical spiking activity of the small intestine occurred as fused spike bursts of large amplitude potentials migrating slowly over short distances only 24 h after feeding. They are interspersed with short periods of irregular spiking activity.

4. These findings suggested that, except the distal part of the small intestine which showed an activity which resembled partially the migrating myoelectric complex observed in other species during the fasting state, the motility patterns of the digestive tract in the cat were not comparable to those observed in the dog or sheep. In the cat, mixing of the contents seemed to result from more or less regular spiking activity allowing their propulsion distally. The propagation over distances varying from 200 to 1000 mm of nine to eighteen daily fused spike bursts in the fasting state remains unclear but they are related to the digestive function in accordance with the displacement aborally of their origin in a prolonged fasting condition.

1. Pancreatic juice was collected from six pigs of 48 kg initial weight fitted with a collection catheter in the pancreatic duct and a return catheter in the duodenum.

2. Measurements of flow and composition of the juice were made during 24 h periods after adaptation to isonitrogenous diets based on barley, wheatings and fish meal (diet BWF) or starch, sucrose, casein, maize oil and cellulose (diet SSC), given in a change-over design. Measurements were also made during the periods of adaptation to a change from one diet to the other.

3. Mean flow-rates for pigs adapted to diets showed a highly significant four-fold difference between diets; values were 4962 ml/d for diet BWF and 1273 ml/d for diet SSC. The hourly volumes of juice were very variable and showed no clear response to feeding and no consistent diurnal pattern for either diet.

4. There were no significant differences between diets in the specific activities of the proteases. Average values were (units/nig protein) trypsin. (EC 3.4.21.4) 29·6, chymotrypsin (EC 3.4.21.1) 7·7, carboxypeptidase A (EC 3.4.17.1) 0·65, carboxypeptidase B (EC 3.4.17.2) 1.6. The total output of each tended to be higher with diet BWF than with diet SSC. The specific activities and tolal outputs of α-amylase (EC 3.2.1.1) and lipase (EC 3.1.1.3) were significantly higher for diet BWF than for diet SSC; specific activities for the two diets respectively were: (units/mg protein) α-amylase 95·6 and 42·3, lipase 59·0 and 14·5.

5. The higher daily volume of juice with diet BWF was associated with significantly (but only slightly) higher levels of both sodium and potassium, compared with diet SSC.

6. The results are discussed in relation to previous studies on digestion at this Institute, in which pigs with intestinal cannulas were given the same diets.

1. Q-strain mice selected for high-line (QLF) or low-line (QSC) body-weights at 6 weeks of age were culled to litters of two or eight (QLF-2, QLF-8, QSC-2, QSC-8) at birth andwere suckled in these groups until 19 d of age.

2. Body-weights were measured daily for all groups and body compositions compared at birth and 19 and 42 d of age. Food intakes and urinary and faecal nitrogen were measured during metabolism trials between 19 and 42 d.

3. QLF-2 and QSC-2 mice grew faster than the corresponding groups of eight until 19 d of age. They also deposited more fat as a percentage of total gain.

4. In the period 19–42 d the influence of genetic selection reappeared and was manifest in a slowing of growth rates of QLF-2 and QSC-2 animals so that by 42 d of age therewere no differences in body-weight between the groups within a line.

5. During the period 19–42 d the total food intakes of each group within a line did not differ although, on the basis of food intake per unit metabolic body-weight (g/kg body-weight0.75 per d) QLF-2 and QSC-2 ate less food than QLF-8 and QSC-8respectively.

6. The differences in body-weight at 19 d between groups were largely overcome by the increased contribution of protein and water to the weight gain of the groups of eight duringthe post-weaning period.

1. Male WAG/Cpb inbred rats fed on rations containing 1·5 mg copper/kg (deficient) and 6·0 mg Cu/kg (adequate) were supplemented with molybdenum (500 mg/kg diet). Starting at week 0 rats were killed weekly for up to 6 weeks and the caeruloplasmin activity of plasma, the Cu concentration of plasma, liver and kidney and the Mo concentration of liver and kidney were determined. The experiment was repeated with rats fed on diets of the same composition but given additional Cu for periods of 2 weeks. Cu was given orally by increasing dietary Cu to 6·0 mg/kg and 25·0 mg/kg for Cu-deficient and Cu-adequate rats respectively or intraperitoneally by injecting 75 μg and 250 μg every second day to Cu-deficient and Cu-adequate rats respectively.

2. After Mo administration to Cu-deficient rats plasma and kidney Cu and liver and kidney Mo increased but caeruloplasmin activity and liver Cu decreased. In Cu-adequate rats plasma, liver and kidney Cu and liver and kidney Mo increased to much higher levels than in Cu-deficient rats. Caeruloplasmin activity was not affected. Fluctuations in plasma Cu and kidney Mo were correlated closely.

3. No qualitative difference between the effect of oral or intraperitoneal Cu administered to Mo-treated Cu-deficient or Cu-adequate rats was found. In Cu-deficient Mo-supplemented rats additional Cu increased plasma Cu, caeruloplasmin activity and liver and kidney Cu and Mo. In Cu-adequate Mo-supplemented rats additional Cu decreased plasma Cu and liver and kidney Mo and increased caeruioplasmin activity and kidney Cu and, to a minor extent, liver Cu.

4. In view of the assumption that in rats a Cu, Mo and S containing compound, related to Cu-thiomolybdate, may be formed in vivo the results suggest thai Cu binds to the Mo-S part of the compound; when this compound is formed in the gastro-intestinal tract it can not be absorbed and when it is formed at systemic sites it changes the Cu distribution.

1. Digesta were collected from twenty-two pigs, of 40 kg mean live weight, and fitted with single re-entrant cannulas in either the duodenum, jejunum or ileum.

2. Three approximately isonitrogenous diets were given to the pigs; their main constituents were: barley, fine wheat offal and white fish meal (diet BWF); starch, sucrose, maize oil, cellulose and either groundnut (diet SSG) or casein (diet SSC).

3. The activities of pepsin, chymotrypsin and trypsin were measured every hour in duodenal digesta during 24 h collection periods. Chymotrypsin and trypsin were also measured every hour in jejunal digesta and every 6 h in ileal digesta, during 24 h collection periods.

4. The mean total pepsin activities in the duodenal digesta during 24 h collection periods (units for a 40 kg pig given 1·7 kg diet) were: 7764400 (diet BWF), 6078400 (diet SSG), 5801 600 (diet SSC).

5. The mean total chymotrypsin activities (units for a 40 kg pig given 1·7 kg diet) in digesta in the duodenum, jejunum and ileum respectively were: 62920, 59560, 21 880 (diet BWF), 78240, 68400, 24680 (diet SSG). 75280, 76120, 6160 (diet SSC).

6. The mean total trypsin activities (units for a 40 kg pig given 1·7 kg diet) in digesta from the duodenum, jejunum and ileum, respectively were: 256840, 362840, 77600 (diet BWF), 211 200, 205280. 46720 (diet SSG) 325720, 428560, 13600 (diet SSC).

7. It was calculated that the total weights of pepsin, chymotrypsin and trypsin in duodenal digesta in 24 h periods were between 6·2 and 7·1 g. This represents 20–25% of previously published estimates of the amounts of endogenous protein in this part of the gut.

1. Balance trials with respiration measurements were performed with twelve rats and twelve pigs given either low- or high-crude-fibre diets. There were six collection periods with the rats over a live-weight range of 86–264 g and three collection periods with the pigs over a live-weight range of 30–55 kg. Measurements were made on the influence of microbial activity in the digestive tract on digestibility and nitrogen and energy metabolism. Dietary inclusion of the antibiotic Nebacitin was the method used to reduce the microbial population.

2. The microbial activity in the hind-gut (μmol ATP/g air-dry contents) of antibiotic-treated rats was reduced to approximately one-tenth of that of untreated rats.

3. Live-weight gain was not significantly affected in either species by a reduction in the microbial activity, in spite of a reduction in dry matter digestibility in animals with reduced microflora.

4. For rats on low-crude-fibre diets, a reduction in microflora reduced digestibility of all nutrients and energy and metabolizability of digestible energy by approximately 5·4%. All differences were highly significant. On high-crude-fibre diets the decrease was approximately 5·9%. In pigs on both crude fibre levels, the digestibility was also influenced by the level of microflora, but the pattern was somewhat different from that obtained with rats, with the Nebacitin treatment increasing the digestibility of N slightly, and the digestibility of fat markedly.

5. Retained N in rats reached a maximum when the rats were approximately 60 d old and thereafter decreased with increasing age. However, for pigs daily N retention increased with age. The retained N:digested N value decreased linearly with age in the rats, but varied little with age over the range (104–146 d) studied in the pigs.

6. The metabolizability of gross energy (metabolizable energy (ME): gross energy) was significantly reduced with an increase in crude fibre level and by the addition of Nebacitin.

7. Retained energy (RE) in relation to ME (RE:ME), was not significantly affected either by level of microbial activity or by crude fibre.

8. The ratio, RE as fat (RF):RE as protein (RP) increased as the animals grew. In the rat experiment there was a tendency for RP to be higher for animals with normal microflora than for animals with reduced microflora for both crude fibre levels.

9. With rats, the regression analyses indicated that the energy requirement for maintenance could be influenced by both the level of microbial activity in the digestive tract and by the level of fibre in the diet. The net availability of ME for maintenance and growth by rats averaged 0·72 for all treatments.

10. The net availability of ME for growth in the pigs averaged 0·65 for all treatments.

1. The effects of three levels of linseed oil (LSO) supplementation of a basal diet on rumen digestion and flow of nutrients to the proximal duodenum of three mature sheep provided with permanent rumen and duodenal re-entrant cannulas were studied.

2. A basal diet of 200 g hay and 400 g concentrates daily, providing approximately 7.0 MJ digestible energy and 13 g N/d, was given alone or with supplements of 13, 26 or 40 ml LSO/d in two equal portions at 06.00 and 18.00 hours. The flow of duodenal digesta was measured by spot-sampling using chromic oxide paper as the marker. Bacterial protein synthesis (BPS) was measured by the diaminopimelic acid technique.

3. Addition of LSO reduced the digestion of energy and organic matter, particularly acid-detergent fibre, in the stomach. Digestion in the intestines increased but at the higher levels of supplementation this failed to compensate completely for the reduction in rumen digestion. Total volatile fatty acid concentrations were not affected but molar proportions of acetate and butyrate were decreased by approximately 18 and 61% respectively while the molar proportion of propionate was increased twofold by the highest concentration of oil. The higher concentrations of LSO virtually eliminated protozoa from the rumen.

4. The second increment of LSO (26 ml/d) produced the highest duodenal flow of total N and bacterial N and the highest efficiency of BPS. The highest concentration of oil (40 ml/d) was without effect. Rumen and duodenal ammonia concentrations and plasma urea concentrations tended to be reduced by the higher concentrations of LSO.

5. It is argued that the results support suggestions made elsewhere that free oils reduce the efficiency of BPS but that they also reduce the numbers of protozoa which can cause an increase in the efficiency of BPS. The net effect of free oil supplementation on BPS is thus likely to be variable and difficult to predict.

1. Addition of supplemental choline to a biotin-deficient diet decreased the biotin status of chicks and increased mortality from fatty liver and kidney syndrome (FLKS).

2. Mortality was also increased by dietary supplementation with a mixture of other B-vitamins, excluding biotin, and was highest when the choline and B-vitamin supplements were combined.

3. The occurrence of sudden death syndrome (SDS) was unaffected by dietary biotin concentration.

4. A previously unreported condition was observed in which birds died showing post-mortem signs characteristic of both FLKS and SDS and whose occurrence was related to the biotin status of the chicks.

1. Six sheep, each fitted with a rumen cannula and re-entrant cannulas in the proximal duodenum and distal ileum, were given two diets (600 g dry matter (DM)/d) consisting of either grass silage (32·1 g nitrogen/kg DM) or dried grass (18·3 g N/kg DM). A net loss of N occurred between mouth and duodenum with the silage diet, indicating extensive ruminal degradation of dietary N, compared with a net gain on the dried-grass diet. Consequently, despite higher N intakes when silage was given, N flow at the duodenum was similar for both diets.

2. The proportion of microbial N in duodenal digesta N was estimated using diaminopimelic acid (DAPA), [35S]methionine (35S), 15N-enriched non-ammonia-N (15NAN) and amino acid profiles (AAP) as microbial markers. Isotopic labelling of rumen micro-organisms was achieved by intraruminal infusions of Na235SO4 and (15NH4)2SO4.

3. A comparison of all methods was made based on the marker concentrations in microbial fractions isolated by differential centrifuagation of strained rumen contents. With both diets, DAPA gave the highest estimates and AAP the lowest. Estimates based on 35S and 15NAN were intermediate and did not differ significantly (P > 0·05).

4. For the 15NAN, 35S and AAP methods, the effect of site of sampling of the microbial fraction, i.e. from rumen contents or duodenal digesta, was examined and in all instances mean estimates based on duodenally-derived microbes were higher. However, the differences were significant for only 15NAN with both diets (P < 0·001), for 35S with the dried grass (P < 0·05), and for AAP with the silage (P < 0·05). Estimates based on duodenally-derived microbes were higher (P < 0·05) using 15NAN than those obtained using 35S with both diets.

5. Depending on the method used for estimating microbial N, estimates of the efficieny ofmicrobial N synthesis in the rumen (g microbial N flow at duodenum/kg organic matter apparently digested in the rumen) ranged between 16 and 38 for the silage diet and 10 and46 for the dried grass diet. Similarly, estimates of feed N degradability in the rumen ranged between 0·62 and 0·97 for the silage and 0·00 and 0·93 for the dried grass.

1. Protein-bound, trichloroacetic acid- (TCA) insoluble 99Mo appeared in plasma a few hours after the introduction of 99Mo-labelled molybdate (30 mg Mo) into the rumen of sheep maintained on a basic diet supplemented with elemental sulphur (3 g S/d).

2. Most of the 99Mo could be displaced from its protein carrier in vitro and the labelled compounds displaced were identified by Sephadex G-25 chromatography as di- and trithiomolybdate. Tetrathiomolybdate was not detected.

3. In control experiments protein-bound, TCA-insoluble 99Mo predominated in plasma after the direct administration of [99Mo]tetrathiomolybdate, either into the rumen or intravenously. The 99 Mo could be displaced in vitro and [99Mo]tetrathiomolybdate identified, although [99Mo]trithiomolybdate was also present.The study provides direct evidence of thiomolybdate synthesis and absorption in ruminants in vivo.

1. Protozoa were labelled by incubating 100 ml rumen fluid with [14C]choline for 1 h. The protozoa were concentrated by centrifugation and then washed with rumen fluid. This reduced residual 14C in the fluid medium to insignificant amounts whilestill retaining the viability of the labelled protozoa. Washing procedures using formal saline (40 g formaldehyde/1 saline (9 g sodium chloride/1)) and saline were developed to isolate protozoa for estimation of specific radioactivity.

2. The protozoal pool in freshly-collected rumen fluid incubated in vitro retained 90%of the radioactivity for up to 6 h following addition of 14C-I a belled protozoa produced as indicated previously. The specific radioactivity of protozoa did not change during the incubation period.

3. Protozoa labelled with [14C]choline and then stored until they died rapidly lost 14C to methane when they were incubated in rumen fluid or were injected into the rumen. Some [14C]choline was salvaged under these conditions by the live protozoa present as they apparently incorporated up to 13% of the label from the dead protozoa. However, protozoal debris from the injected solution could also have been present in the isolated protozoa.

4. The in vitro results suggested that the protozoal preparations were viable, and that the incorporated choline did not have a turnover in excess of the turnover of nitrogen (i.e. specific radioactivity remained constant with time in vitro) suggesting that the dilution of specific radioactivity of protozoa following mixing of a 14C-labelled dose of protozoa represented the rate of irreversible loss and also replacement of protozoa in the rumen.

5. 14C-labelled protozoa had a half-life in the rumen which was greater than that of rumen fluid and in six animals the protozoal replacement rate was l-4·l mg N/min.

6. Losses of 14C from labelled protozoa in the rumen in methane or via abomasal digesta were 65 and 35% respectively.

7. The results suggest that protozoal growth may be as high as 32% of the total microbial protein synthesis in the rumen but that 65% of the protozoa die and are degraded in the rumen.

1. A method for estimating the proportions of bacterial- and protozoal-N in the total non-ammonia-N reaching the lower gut of the ruminant under steady-state conditions was evaluated. Three trials using two different diets were conducted with a Holstein steer equipped with a rumen cannula and duodenal re-entrant cannulas.

2. An intraruminal primed infusion of (15nh4)2so4 was administered for 68 h during each trial. Bacteria and protozoa samples were isolated from rumen fluid at approximately 6 h intervals during each infusion period. Total non-ammonia-N was isolated from duodenal digesta samples taken at approximately the same times. All of these samples were analysed for 15N enrichment. A computer program was used to fit equations to the 15N-enrichment curves of bacterial- and protozoal-N. Models of both bacterial- and protozoal-N kinetics consisted of a small pool which equilibrated rapidly with rumen NH3 and a large pool with a fractional turnover rate of 0.045–0.070/h for bacterial-N and 0.056–0.069/h for protozoal-N.

3. Abomasal fluid turnover was estimated by a single injection of polyethylene glycol (molecular weight 4000) into the rumen followed by sampling of rumen fluid and duodenal digesta.

4. Estimates of abomasal fluid turnover, bacterial-N turnover, and protozoal-N turnover were entered into an equation which was adjusted by computer iteration to fit the 15n-enrichment curve of duodenal digesta non-NH3-N generated from each (15nh4)2so4 infusion period. The computer fit of this equation to the observed results gave estimates of 0.39–0.45 and 0.22–0.41 for the proportion of duodenal non-NH3-N derived from bacterial-N and protozoal-N respectively.

5. This method is potentially useful in estimating microbial protein passage to the lower gut in ruminants. Sampling digesta from the omasum rather than the duodenum would simplify the method and possibly increase the reliability of the estimates.

1. The deposition of fat during pregnancy and its subsequent mobilization during lactation was studied in the rat. The utilization of protein during lactation was also investigated.

2. Fat accumulation was rapid and continuous from early pregnancy. Approximately one-third was deposited subcutaneously, the remainder being stored in central depots.

3. The volume of milk produced during lactation was varied by offering protein at two concentrations from day 2 post partum. Optimum lactation was achieved with a high-protein diet. The dams lost no body protein, but 60% of the total body fat was mobilized, despite a marked increase in food consumption. On a low-protein diet, food intake did not rise during lactation. Some body protein was catabolized and, again, 60% of the fat was mobilized although milk output, as judged by the gain in carcass energy of their pups, was reduced to approximately one-third of optimum.

4. Protein was used more efficiently by lactating rats, when compared with virgin controls.

5. We conclude that fat stored during pregnancy makes a major contribution to the energy costs of lactation, and that its mobilization is under hormonal rather than dietary control.