The aim of the study was to measure the secretion, passage and reabsorption of N in the gastrointestinal tract of sheep using the method of digesta exchange between ¹⁵N labelled and unlabelled sheep. This requires information on N passage in different parts of the intestinal tract. Six experiments were carried out, each with three male sheep of 20-25 kg body weight, fitted with a cannula into the rumen, with re-entrant cannulas in the proximal duodenum and distal ileum and with a jugular vein catheter for blood sampling. The animals were fed in 4 h intervals with a hay and concentrate diet, where Group 1 (Experiments 1 to 3) received a ration of low crude fibre (15% CF) with a hay:concentrate ratio of 38:62, and Group 2 (Experiments 4), a ration of high crude fibre (25% CF) with a hay:concentrate ratio of 64:36. The diets were isonitogenous (~16% CP). In each experiment one of the three sheep (animal No. 1) was infused intraruminally with ¹⁵N urea (1 g/d, 95 atom% ¹⁵N ) , the others, No. 2 and No. 3, were infused with unlabelled urea. After the ¹⁵N level of the metabolic pool of animal No. 1 reached a steady state, passage of digesta, dry matter, total N, and ¹⁵N at the duodenum and the ileum and N and ¹⁵N excretion in faeces and urine were estimated. For the determination of N secretion and reabsorption, on day 7 and 8 of the experiment the duodenal and the ileal digesta were exchanged between the labelled animal (No. 1) and the unlabelled ones (No. 2 and 3). The digesta flow was measured directly during 48 h and 3 % aliquots of the duodenal and ileal digesta were taken for analysis of N and ¹⁵N content. Mean N balance (± SD) for Group 1 (low CF content) was 5.80±1.66 g N/d and for Group 2 (high CF content) - 1.11±1.12 g N/d. Total N excretions for Group 1 were significantly smaller than for Group 2 (urine: 43 vs 71 % of N intake; faeces: 27 vs 38% of N intake). Mean daily N excretions in faeces were 5.21 and 4.86 g for Groups 1 and 2, respectively and were positively correlated (R² = 0.912) with CF intake. For determination of N flow rates out of the stomachs into the duodenum it was necessary to correct the flow rates measured at the duodenal fistula by the secretion rates of pancreas and bile. These secretion rates, estimated in a separate experiment, were 2.20 and 0.93 g N/d for Groups 1 and 2, respectively. Corresponding N flows out of the stomachs into the duodenum were 12.2 and 11.6 g/d and were higher than the N intake for the group with higher CF content. Ileal N flow was positively correlated to CF intake (R² = 0.819). For Groups 1 and 2, N disappearance rates were 57.9 and 45.6%, respectively for the small intestine, 26.5 and 30.1% for the large intestine, and 76.5 and 62.2% for the whole digestive tract.