After the oral and intraperitoneal administration of urea-¹⁴C and sodium bicarbonate-¹⁴C to rats, the radioactivity of carbon dioxide expired in breathing air and that of urea transported into the blood were measured. Ureolysis was examined quantitatively on the basis of a compartment model analysis, where the hydrolysis and transport of substances were assumed to be first-order processes. Ureolysis in vivo was found not only in the lumen, but also in other tissues such as juxtamucosa, kidney and liver ; the former rate constant (h^-1) was 0.759 (k) and the latter 0.0122 (k'). The constant k observed in normally fed rats was 9.5-fold higher than k in fasted rats, but k' was low in both cases. The administration of nicotino-hydroxamic acid as a urease inhibitor completely depressed k, but k' was not affected. On the bases of 71.0 mg·kg^-1·h^-1 of urea excreted in urine and the rate constants obtained for transport and hydrolysis of urea, the amounts of urea hydrolyzed in the lumen and other tissues were estimated to be 4.92 and 5.04 mg·kg^-1·h^-1, respectively. Consequently, it was calculated that urea hydrolyzed in the body corresponded to 12.3% of urea synthesized in the liver.