Two sheep weighing about 25 kg were fed on rations with similar nitrogen content (about 9 g N/day including 15N from 5 g labelled urea) but with different content of readily digestible carbohydrate. Concentrate ration consisted of maize meal (whole plant) 300 g + barley meal 100 g + mineral mixture 20 g + urea 5 g; fibrous ration--meadow hay 250 g + oat straw 170 g + mineral mixture 20 g + urea 5 g. Retention of 15N was 53.8 and 43.6% of intake in sheep on concentrate or fibrous diet, respectively. Incorporation of 15N into individual amino acids of bacterial protein isolated from the rumen and blood plasma proteins was different, depending on the ratio and amino acid. Enrichment of 15N of the bacterial matter and plasma proteins in both sheep exceeded that of individual amino acids, indicating that urea nitrogen was utilized to a large extent for the synthesis of nitrogen compounds other than amino acids.