1. 15N-labelled mixed rumen bacteria, obtained from a steer that had received [15N]urea in its diet, were disrupted ultrasonically and freed from nucleic acids and their degradation products. Samples were subjected to a simulated abomasal digestion with pepsin.

2. The digests were infused with a non-absorbable marker (polyethylene glycol) into the duodenum of four steers equipped with simple duodenal and re-entrant ileal cannulas and adapted to a diet of barley straw, flaked maize and urea. The outflow from the ileum was collected for 6–7 h. The mean value for the digestibility of 15N bacterial proteins in the small intestine was estimated to be 0.74.

3. [14C]urea was administered intravenously during the infusion of the 15N-labelled protein into the duodenum. Urine and faeces were collected for the next 48 h and the proportion of urea-N produced, that was excreted in the urine, estimated from urine 14C excretion. Total urea 15N production was estimated from this value and the amount of 15N excreted in the urine. The mean proportion of 15N absorbed that was deposited in body protein, 0.70, was calculated by difference. The over-all efficiency of utilization of 15N in the infused rumen bacterial protein was 0.52.

4. An approximate estimate of the mean rate of protein synthesis calculated from the data was 24 g/kg body-weight (W)0.75 per d and compared with an estimated net deposition of protein of 1.67 g/kg W0.75 per d.

5. The importance of these values in factorial schemes for estimating ruminant N requirements is discussed.

1. Two experiments were conducted to evaluate the effects of caecectomy and source of dietary fibre and starch on excretion of amino acids by laying hens. In Expt 1, caecectomized hens excreted significantly higher levels of several amino acids than intact hens during a 24 h fasting period. These differences were greatest for threonine, proline and leucine.

2. In Expt 2, caecectomized and intact hens were given either a nitrogen-free diet containing 500 g cellulose/kg or one containing 400 g uncooked potato starch/kg and 100 g citrus pectin/kg. The potato starch-pectin (PSP) diet increased excretion of most amino acids by both caecectomized and intact hens compared with the cellulose diet, with this response being larger for intact hens. When compared across diets, caecectomized hens excreted more threonine and serine than intact hens.

3. Excreta voided by hens given PSP contained higher levels of alanine and valine and lower levels of aspartic acid, glutamic acid and methionine than excreta from the cellulose diet. When compared across diets, excreta from caecectomized hens contained more threonine, serine and isoleucine and less aspartic acid and alanine than excreta from intact hens.

4. Multiple regression analyses of excreta amino acid profiles on profiles of endogenous and microbial protein suggested that the intestinal microflora had greater influence on amino acids excreted by caecectomized hens than on those excreted by intact hens.