Sheep fitted with a PVC cannula in the dorsal rumen, a silastic infusion tube in the abomasum, and single ‘T’-shape PVC cannulas in the proximal duodenum (D1), distal duodenum (D2), mid-jejunum (J), and terminal ileum (I), were fed on diets varying in crude protein (Nx6·25) and fibre contents. 15N was administered to the sheep as a 15N-labelled grass meal-beer yeast suspension which was continuously infused into the abomasum over a period of 1 week. Subsequently, over a period of 4 d, 15N excess in the D1, D2, J and I digesta and faeces was measured and related to 15N excess in blood plasma TCA-soluble N. Average daily intakes of DM and N were 1·12 kg and 26·3 g respectively. Average daily endogenous N (g) amounted to 3·1 at D1, 10·2 at D2, 6·1 at J, 5·0 at I, and 5·1 in the faeces (F). Corresponding percentage values for total N were: D1 12·4, D2 32·9, J 46-4, I 48.4 and F 50.7. The secretion, particularly of pancreatic juice and/or bile, varied. The apparent re-absorption of endogenous protein varied among experimental diets, on average by 50·4% between D2 and I. Along the small intestine the percentage of 15N in the TCA-precipitable fraction gradually changed, on average by 66·0, 63·0, 46·3 and 55·7 at D1, D2, J and I respectively. Because of differential rates of enrichment and turnover among protein pools which contribute towards endogenous secretion, care should be taken in interpreting these data as absolute values.