The distribution, metabolism and excretion of the radioactivity were studied in male rats after the bolus intravenous administration of 14C-MT-141. The biological half-lives obtained from the blood concentration-time curve were 0.43 hour for the data in the first 4 hours and 16.5 hours for the data from 6 hours to 24 hours after the intravenous administration of 14C-MT-141. The radioactivity was excreted mainly into urine, and the cumulative urinary and fecal excretion of the radioactivity were 75.2% and 24.1% of the dose, respectively, within 120 hours after the intravenous administration of 14C-MT-141. The cumulative biliary excretion of the radioactivity was 18% of the dose within 48 hours after the intravenous administration, and 35% of the radioactivity excreted into bile (about 6% of the dose) was reabsorbed from the intestine. The highest concentration of the radioactivity was observed in the kidneys, and also the relatively high concentrations were observed in the liver, plasma and intestine, while the concentrations in the brain, fat and muscle were low. Within 24 hours after the intravenous administration of 14C-MT-141, the radioactivity in the highly distributed organs or tissues was decreased to less than 3% of the values at 5 minutes after the intravenous administration. A small amount of N-acetyl-MT-141 was found in urine and feces as a metabolite.