1. 14C and 15N isotopes of urea were infused intravenously into rabbits for 6–8 h in order to measure urea synthesis and the extent of degradation in the digestive tract. The results indicate that 0.62 of the urea flux was excreted in the urine and that re-incorporation of urea-N following hydrolysis in the gut represented 0.3 of the urea synthesis rate.

2. Sampling of metabolites from the caecum by dialysis provided an opportunity to assess the contribution of urea-N to the caecal ammonia pool. This contribution is calculated to be 0.25 of caecal ammonia turnover.

3. Infusion of a urease (EC 3. 5. 1. 5) inhibitor during a continuous infusion of [14C]urea into the caecum permitted the measurement of urea turnover within the caecum.

4. Results obtained for urea entry into the caecum are contrasted with the measured urea degradation rate in the gut.