Absorption, distribution, excretion and metabolism were studied in rats and mice after oral and intravenous administration of 5 mg/kg of [14C] T-3761. 1. [14C] T-3761 was rapidly absorbed after oral administration and maximum serum concentration of radioactivity was observed at 15 min. and at 10 min. in rats and mice, respectively. After that, radioactivity of serum declined rapidly. 2. The administered radioactivity was excreted mainly in the urine in rats and mice and excretion rate was 84% and 74%, respectively. The residual radioactivity was excreted in the feces. In both species, the urinary excretion rate after oral administration was similar to that after intravenous administration, suggesting that [14C] T-3761 was absorbed almost completely. 3. In the rat urine, over 90% of excreted radioactivity was T-3761. On the other hand, about 57% of excreted radioactivity was presumed to be glucuronide conjugate of T-3761 in the mice urine. 4. The biliary excretion of radioactivity in rats after oral administration was approximately 22%, and about 83% of those was presumed to be glucuronide conjugate. About 40% of the excreted radioactivity was reabsorbed from the intestinal tract. 5. Except gastrointestinal tract, tissue concentration of radioactivity after oral administration was high in kidney and liver. Radioactivity was widely distributed in other tissues at lower concentration than in the plasma, whereas hardly distributed in brain and spinal cord. 6. Whole body autoradiograms in rats indicate that radioactivity were widely distributed except central nervous systems. 7. The extent of the binding of [14C] T-3761 to serum protein was 23-31% and 25-34% in rats and mice, respectively.