1. When rumen fluid together with small quantities of (15NH4)2S04 [1-14C]sodium acetate and polyethylene glycol (PEG) was introduced into the emptied rumen of a sheep, no significan change occurred in the ratio 15N:14C during a period of 30–60 min. The ratio 15N:PEG, however, declined by 22–53%. when(15NH4)2SO4, [1-14C]sodium acetate and PEG were introduced into the normal rumen of a sheep fed on lucerne hay the ratio 15N:14C in the whole rumen contents showed no appreciable change in a period of 1 h, but the ratio 15N:PEG declined by 25–41%. It was therefore concluded that the rate constants for removal of ammonia nitrogen (NH2-N) and volatile fatty acid (VFA) from the rumen were very nearly equal.

2. In sheep fed at 1 h intervals on lucerne hay chaff or lucerne hay pellets VFA production, VFA and NH3-N concentration in the rumen fluid and the volume of fluid passed from the rumen to the omasum were determined. From these results and the information previously gained on the relative rates of removal of NH2-N and VFA from the rumen, the amounts of NH3-N produced in the rumen and absorbed there or at lower levels of the digestive tract were calculated to be equivalent to 23–27% of the dietary N in lucerne hay chaff and 17% of that in lucerne hay pellets; 59–66% of the NH3-N was absorbed from the rumen and the remainder passed to the omasum.

1. The effect of riboflavine deficiency on liver ribonuclease activity, RNA and DNA content, and 32P incorporation into RNA and DNA has been studied in rats maintained on a 16% protein diet, a protein-free diet and on a protein-free diet subsequently replaced with a 40% protein diet.

2. Rats maintained on a riboflavine-deficient diet for 45 days showed decreased incorporation of 32P into liver RNA but no effect on the RNA content of liver. The concentration of DNA in liver and 33P incorporation into it remained unaffected. After a deficiency period of 70 days, both the RNA and DNA contents of liver were found to be decreased. When the riboflavine-deficient or control rats were given the protein diet for 30 days and then a proteinfree diet for 15 days, the RNA content of their livers decreased, while the liver DNA content was increased. Repletion with a 40% protein diet restored the RNA and DNA content in both control and riboflavine-deficient rats.

3. Liver ribonuclease activity was decreased after a deficiency period of 45 days, whereas it was increased after a deficiency period of 70 days.

4. A correlation between liver RNA level and liver ribonuclease activity in riboflavine deficiency is suggested.

1. Weanling rats were fed on an iron-deficient diet without added folic acid and litter-mate controls received the same diet supplemented with Fe. Groups of deficient and control animals were killed at intervals of up to 88 days and glutamate formiminotransferase activity in liver and folic acid activity in liver and serum were measured.

2. No differences were found between the results from the deficient and control animals, although the deficient diet produced marked falls in haemoglobin, packed cell volume and serum iron.

3. In a second experiment folic acid was added to the diets and twelve rats received the Fe-deficient diet and their litter-mates were fed on the Fe-supplemented diet for 91 days. No differences were found in the activity of glutamate formiminotransferase purified from the combined livers of the two groups.

1. Procedures, based on those of Schmidt & Thannhauser (1945) and Schneider (1945), for the extraction and estimation of nucleic acids in bovine digesta were examined in detail.

2. Final methods which were suitable for routine determination of RNA and DNA were essentially as follows. Digesta samples were extracted in the cold, first with a solution of trichloroacetic acid in ethanol, then with aqueous trichloroacetic acid solution and finally with lipid solvents. The dried residue was hydrolysed with alkali, purified by passage through a Dowex resin, and the RNA, in the form of mononucleotides, determined by U.V. absorption. DNA was determined separately in hot perchloric acid extracts of the original dried residue by colorimetric estimation of the deoxyribose content.

1. A study was made of the partition of the major constituents of rat's milk during the formation of the clot in the infant rat's stomach.

2. Milk was obtained from the dams by manual expression after separation from their litters for periods ranging from 12 to 23 h.

3. The ratio of fat to protein in the clots removed from the stomachs of the 11-day-old pups was more than double that found in the milk. The difference was due to the high fat content of the clot.

4. The stomach contents of DUDS which were allowed to suckle for I h following a 12 h separation from their dams had fat to protein ratio similar to that of the expressed milk, When suckling was prolonged, the ratio reverted to that of the established clot.

5. Milk secreted under conditions of uninterrupted lactation and promptly recovered from the stomachs of pups previously fasted had a value for fat to protein comparable to that for the clot from a normally fed pup. An interruption in suckling therefore reduced the fat content of the milk.

6. Part of the carbohydrate in rat's milk is secreted in a bound form. Lactose accounts for approximately one half of the total carbohydrate. Evidence is presented which suggests that the remainder may be an isomer of lactose.

7. Fat and lactose undergo hydrolysis to a small extent only in the pup's stomach. The breakdown of protein is insignificant. Rat's milk contains no proteolytic enzymes.

8. No significant differences in composition were found in the outer shell, the core and pyloric regions of the clot. Thus fat and casein leave the stomach in constant proportion.

1. Six adult, non-pregnant, non-lactating, Friesian cows were used when fat and when thin to measure differences in the voluntary intakes of straw, hay and hay plus concentrates caused by the fatness of the animals. Measurements of digestibility, time of retention of food in the digestive tract, rate of breakdown of cotton threads in the ventral sac of the rumen and amounts of digesta in the reticule-rumen were included.

2. The mean voluntary intakes of straw were similar for fat and thin cows. In absolute terms, thin cows consumed 31 % more hay and 23 yo more hay and concentrates than fat cows; in relation to metabolic body size (W0.75), these differences were 76% and 52% respectively.

3. Small decreases in digestibility of these diets by the thin cows, reflected in slight reductions in the rate of loss of weight of cotton threads placed in the rumen, did not alter the significance of the differences in intake between fat and thin cows.

4. Small changes in time of retention of food in the digestive tract suggested that the capacity of the tract may have been greatest in the thin cows.

5. The presence of a greater amount of digesta in the reticulo-rumen of thin cows than in that of fat cows after eating hay supports the suggestion of a greater gut capacity in these animals. In both fat and thin cows, the capacity of thereticulo-rumen didnot appearto havelimited the intake of the hay and concentrate diet. In both groups the lowest levels of rumen fill were observed after straw was given.

6. The results are discussed in terms of possible mechanisms which may operate to reduce the voluntary intake of medium- and good-quality diets as cows become fatter. When poorquality roughages are given, other factors appear to conceal any differences in intake which may exist between fat and thin cows.

1. Experiments were undertaken to study the effect of daily intraperitoneal injection of acetoacetate for 90 days on vitamin B6 status in male albino rats. The initial dose of acetoacetate was 50 mg per kg body-weight, which was increased by 50 mg per kg body-weight every 15 days.

2. Urinary excretion of vitamin B6 was found to decrease after 30 days in acetoacetatetreated rats. After 75 days urinary values of vitamin B6 were considerably lower in such rats than in the corresponding control rats.

3. When acetoacetate injections were stopped after 90 days and the rats were fed L-tryptophan (100mg per rat), they were found to excrete significantly greater amounts of urinary kynurenine, hydroxykynurenine and xanthurenic acid than the corresponding controls.

4. Blood and liver vitamin Be levels were found to be lower in rats treated with acetoacetate for 90 days than in the untreated rats.

1. The fatty acid compositions of the plasma cholesteryl esters, phospholipids, triglycerides and unesterilied fatty acids were determined in two sheep at various times after they had been given intraruminal infusions of emulsions of maize oil or linoleic acid.

2. The concentration of linoleic acid in the plasma triglycerides began to increase 3 h after infusion began. The infusions of maize oil and linoleic acid resulted in the appearance of peak concentrations of linoleic acid in the plasma triglycerides 6 and 12h respectively after infusion began.

3. After the infusion of maize oil the plasma triglycerides showed an increasein theconcentration of stearic acid but after the infusion of linoleic acid the plasma triglycerides showed an increase in the concentration of oleic acid.

4. The concentration of linoleic acid in the plasma phospholipids and cholesteryl esters did not begin to increase until 6–9 h and 24–25 h respectively after the infusions had begun.

5. No evidence was found for an absorption mechanism which involved the direct incorporation of linoleic acid into the blood phospholipids or cholesteryl esters.

1. Four wether sheep were maintained on a diet of hay for 2 weeks and then starved for a period of 4 days.

2. Immediately before and during starvation the urinary excretion in the following fractions was determined: hippuric acid, creatinine, total diethyl ether-soluble acids of hydrolysed and unhydrolysed urine, total aromatic acids in hydrolysed and unhydrolysed urine and the proportion of the former present as benzoic and phenylacetic acids.

3. A method for determining the benzoic acid content of light petroleum extracts of urine has been developed and is described.

4. Starvation had little effect on the urinary excretion of phenylacetic acid or creatinine, but during the first 2 days of starvation there were large decreases in the excretion of all the other urinary fractions studied.

5. Of the fractions examined, 43% of the diethyl ether-soluble acids of hydrolysed urine and 42% of those of unhydrolysed urine were of exogenous origin; 76% of the total urinary aromatic acids were of exogenous origin. Partition of the aromatic acids in the urine of two of the four sheep indicated that the reduction in aromatic acid excretion on starvation was completely accounted for by the decline in benzoic acid output. Almost all the hippuric acid (97%) was of exogenous origin.

6. These results have been compared with the urinary output of aromatic acids by nonruminants when fasted, and possible reasons for the relatively large amounts of phenylacetic acid found in the urine of starved sheep have been discussed.