Urea providing from 1 to 15 g N/day was continuously infused per rumen or per abomasum into sheep consuming a diet of chopped wheaten hay. Blood urea was directly proportional to the quantity of urea N infused. The infusion of urea per abomasum produced an increase in the concentration of ammonia in rumen liquor due to an enhanced transfer of N from the blood to the rumen. By comparing the increments in rumen ammonia concentration produced by urea infusions per abomasum and per rumen, it was shown that when 2.5 g or less of urea N were infused per abomasum, transfer of N to the rumen increased by an amount similar to that infused; increase in the quantity infused beyond approximately 3.5 gN/day, however, failed to increase N transfer further. This limit to N transfer was reached when the concentration of blood urea N was 16-18 mg/100 ml. It was calculated, after making certain assumptions, that the maximum amount of N transferred to the rumen was 4-5 g/day, the equivalent of 1.5-1.8 times the body urea N pool at blood urea N levels of 16-18 mg/100 ml.