1. An experiment was conducted with three sheep maintained entirely by intragastric nutrition to estimate the digestibility of isolated individual constituents and amino acids (AA) of rumen micro-organisms (RMO) in the small intestine.

2. Five levels of RMO were infused into the abomasum. The apparent and true disappearance of the individual components were measured by regression of abomasal input on the passage at the ileum.

3. The true digestibility values of N, AA-N, DNA and RNA were 0·82, 0·85, 0·81 and 0·87, respectively.

4. The digestibility of individual AA varied between 0·80 and 0·88, the only exceptions being diaminopimelic acid (0·37), histidine (0·68) and cystine (0·73), which were significantly lower than the average (0·847).

5. The endogenous components in the ileal fluid in sheep given protein-free infusions, expressed in mg/kg live weight0·75 per d, were total N 42 and AA-N 20.

1. The metabolic enzymes alkaline phosphatase (EC 3. 1. 3. 1), acid phosphatase (EC 3. 1. 3. 2) and isocitrate dehydrogenase (EC 1. 1. 1. 42) were measured in mucosal homogenates and these enzymes, together with glucose-6-phosphatase (EC 3. 1. 3. 9), were measured in homogenates of isolated enterocytes from germ-free (GF) and conventional (CV) chicks which were either fed continuously until they were killed or were subjected to a 16 h fast before killing.

2. The intestine of the GF chicks was generally lighter than that of the CV controls. The activity of alkaline phosphatase was greater in the mucosal homogenates of the CV chicks compared with the GF birds, but the concentrations of acid phosphatase and isocitric dehydrogenase were not different in the two groups. In the isolated enterocytes the concentration of all enzymes expressed per mg DNA, except alkaline phosphatase, was higher in the GF chicks. Expressed per mg protein there was no significant difference in enzyme activity in the two groups.

3. Fasting caused a reduction in intestinal weight and total mucosal protein in both groups but the reduction was greater in the GF chicks compared with the CV controls. In the GF chicks, fasting caused a significant fall in acid phosphatase and isocitric dehydrogenase activities of the mucosal homogenate, whereas in the CV chicks only acid phosphatase fell to a significant extent. In the isolated enterocytes feeding caused a marked fall in protein per mg DNA in the CV chicks; fasting tended to reduce enzyme concentrations in the GF chicks but to have less effect in the CV group except for alkaline phosphatase where there was a marked rise in activity.

4. It is suggested that the difference in enzyme activities in the mucosal homogenates and isolated enterocytes might result from (a) the presence of a much greater lamina propria in the CV compared with the GF chicks and (b) the greater mitotic activity in the fed CV chicks yielding a much larger number of smaller immature cells.

1. To estimate the tryptophan requirement of the kitten, six male and six female kittens were presented diets containing 0·7, 0·9, 1·1, 1·3, 1·5 and 3·0 g tryptophan/kg diet for six experimental periods lasting 10 d in accordance with a 6 × 6 balanced Latin-square design.

2. Mean daily weight gain and nitrogen retention (N intake minus urinary and faecal N) plateaued at 1·1 g tryptophan/kg diet indicating that the minimal tryptophan requirement of the kitten was 1·1 g/kg diet.

3. Plasma tryptophan concentration exhibited a marked increase at dietary tryptophan concentrations above 1·3 g/kg diet.