Two sheep were fed a diet containing urea and high level of easy fermentable carbohydrate or crude fibre. Incorporation of urea 15N into the total and bacterial N in rumen and NPN of blood plasma was higher, but lower into plasma protein in the sheep fed the diet with easy fermentable carbohydrate. The analysis of data of the rate of enrichment with 15N in protein and NPN of plasma and rumen liquid suggests that urea nitrogen was fixed transiently into plasma protein and after releasing and recirculation throughout the rumen incorporated permanently into plasma protein.