1. To obtain a quantitative model for nitrogen pathways in sheep, a study of ammonia and urea metabolism was made by using isotope dilution techniques with [15N]ammonium sulphate and [15N]urea and [14C]urea.2.Single injection and continuous infusion techniques of isotope dilution were used for measuring ammonia and urea entry rates. 3. Sheep were given 33 g of chaffed lucerne hay every hour; the mean dietary N intake was 23.4 g/d. 4. It was estimated that 59% of the dietary N was digested in the reticulo-rumen; 29% of the digested N was utilized as amino acids by the micro-organisms, and 71 % was degraded to ammonia. 5.Of the 14.2 g N/d entering the ruminal ammonia pool, 9.9 g N/d left and did not return to the pool, the difference of 4.3 g N/d represented recycling,largely within the rumen itself (through the pathways : ruminal ammonia > microbial protein > amino acids > ammonia). 6. Urea was synthesized in the body at a rate of 18.4 g N/d from 2.0 g N/d of ammonia absorbed through the rumen wall and 16.4 g N/d apparently arising from deaniination of amino acids and ammonia absorbed from the lower digestive tract. 7. In the 24 h after intraruminal injection of [15N]ammonium salt, 40-50 % of the N entering the plasma urea pool arose from ruminal ammonia; 26% of the 15N injected was excreted in urinary N. 8. Although 5-1 g N/d as urea was degraded apparently in the digestive tract, only 1.2 g N/d appeared in ruminal ammonia; it is suggested that the remainder may have been degraded in the lower digestive tract. 9. A large proportion of the urea N entering the digestive tract is apparently degraded and absorbed and the ammonia incorporated in the pools of nitrogenous compounds that turn over only slowly. This may be a mechanism for the continuous supply to the liver of ammonia for these syntheses. 10. There was incorporation of 15N into bacterial fractions isolated from rumen contents after intraruminal and intravenous administration of [15N]ammonium salts and [l5N]urea respectively. 11. A model for N pathways in sheep is proposed and, for this diet, many of the pool sizes and turn-over rates have been either deduced or estimated directly.