The retention and excretion of intrajejunally administered 15N-urea was studied in four experiments on two sheep with a permanently fistulated small intestine. In the first 7 days after the administration of 2 g 15N-urea, 18.26% was excreted in the faeces and 19% in the urine; 62.74% was retained in the organism. Urinary excretion took place mainly on the first day and from the 3rd to the 7th day no 15N was present in the urine. The rate of 15N excretion in the faeces was roughly the same for the first 4 days and then fell; on the 7th day there was no 15N in the faeces. The proportion of 15N-urea retained in the organism and excreted in the urine was 81% showing that urea in the ruminant gastrointestinal tract is largely linked up into metabolic circulation as part of the general exchange of nitrogenous compounds.