Urea turnover and the proportion of endogenous urea secreted and excreted in the saliva, the bile, the pancreatic juice and the urine and directly across the wall of the digestive tract was studied in 6 experiments, after a single i.v. dose of labelled 15N, in two adult sheep weighing 49 and 50 kg, with permanent biliary and pancreatic fistulus and with an exteriorized right parotid duct. It was found that, of the total amount of endogenous urea secreted into the animals' digestive tract (0.2694+/-0.0138 mg/min/kg b.w.), 10.27+/-0.94% reached the contents in the saliva, 2.12+/-0.28% in the bile and 0.66+/-0.08% in the pancreatic juice, and that 86.95+/-2.1% was secreted into the gastrointestinal tract, across its wall, from the blood capillaries. Exogenous turnover amounted to 0.3228+/-0.192 mg/min/kg. Of the total amount of 476.6 mg i.v. injected 15N urea, 274.1+/-8.86 mg was excreted in the urine 5.1+/-0.9 mg in the bile, 3.19+/-0.06 mg in the pancreatic juice, 4.96+/-0.76 mg via the right parotid gland and 9.34+/-1.09 mg in the faeces. The results show that the quantitatively most important part of the recirculation of endogenous urea is its passage from the blood across the wall of the gastrointestinal tract into its contents.