Using a combination of fistulation and the ¹⁵N isotope technique, the flow rates of endogenous N secreted postruminal into different sections of the digestive tract were estimated in growing sheep by digesta exchange between ¹⁵N-labelled and unlabelled animals. To investigate the influence of fibre (CF) on these flow rates the estimations were made on two groups that received approximately isonitrogenous diets (16% CP in DM) differing in crude fibre content (Group 1: 14.7%, Group 2: 24.9% CF in DM). The passage rates of total N and ¹⁵N at the duodenum and ileum and the excretion in faeces were measured directly. These data were used to calculate the passage of postruminal N secreted in the different intestinal sections. The microbial N passage at the proximal duodenum and distal ileum was calculated as the product of RNA passage at the duodenum and ileum, and the N:RNA ratio of bacteria isolated from rumen fluid. The flow rates of endogenous N into the duodenum and through the ileum and the excretion rate in faeces were estimated by measuring the appearance and disappearance rates of ¹⁵N in appropriate intestinal sections of the unlabelled animals receiving ¹⁵N-labelled digesta or in the labelled animals getting unlabelled digesta by digesta exchange. The microbial flow rate through the duodenum was in average 5.8 g N/d in Group 1 and 5.1 g N/d in Group 2 (50 and 46% of NAN flux), and 0.9 g N/d (Group 1) to 1.3 g N/d (Group 2) through the ileum (19 and 22 % of NAN flux). The portion of rumen undegraded protein-N of NAN flow was in Group 1 47% both in the duodenum and in the ileum. In Group 2 it was 42% . The endogenous NAN entering the duodenum was low for Group 1 (0.40 g N/d = 3.4% of NAN) and approximately three-fold higher for Group 2 (1.3 g N/d = 12% of NAN). The corresponding values for endogenous N at the ileum were 1.6 g/d (34% of NAN) and 2.2 g/d (36% of NAN) , indicating that in relative terms, endogenous N secretion in this section of the digestive tract was independent of the feeding regime. The excretion rate of endogenous N in faeces varied between the individual experiments (1.84 to 2.39 g N/d for Group 1; 1.54 to 2.00 g N/d for Group 2). In relation to total faecal N excretion the endogenous fraction was higher for Group 1 (56%) than for Group 2 (40%). During all of the experiments, endogenous N as a proportion of total N increased from the duodenum to the rectum. Despite some reservations about the ¹⁵N dilution method, its use together with the technique of digesta exchange between labelled and unlabelled animals is a potentially useful way to quantify the passage of postruminal secreted N in dependence of nutritional factors. Further studies are needed to refine the methodology.