For the purpose of clarifying the pharmacokinetics of esmolol in rats, the plasma level, tissue distribution, metabolism and excretion were investigated after intravenous administration of 14C-esmolol, and the following results were obtained: 1. After intravenous administration to rats, the radioactivity in plasma decreased biphasically, and disappeared almost completely by 24 hr. The elimination half-lives were dose independent (10, 20, 40 mg/kg), and the plasma levels and AUC increased in proportion to the administered doses. The unchanged drug was not detected in plasma at 2 min after administration, but ASL-8123, the hydrolyzed metabolite of esmolol, was accounted for more than 80% of radioactivity in plasma. This indicated esmolol to be rapidly metabolized by esterases in blood. During repeated administration (for 7 days), the radioactivity in plasma showed no accumulation, and pharmacokinetic parameters after the final administration were similar to those after a single administration. 2. The radioactivity distributed rapidly and widely to tissues, and the levels were highest in kidney and lung, followed by adrenal gland, thyroid, pancreas, harderian gland, pituitary, liver and small intestine. The time course profiles in radioactivity levels in the heart, a target organ, were about 2 times higher than those in blood. In whole body autoradiograms, the transfer of radioactivity to the urine was already observed at 5 min after administration, indicating a rapid excretion of radioactivity to the urine. 3. Plasma protein binding of radioactivity in vitro was about 23%, and that in vivo (from 5 min to 8 hr after administration) accounted for 22-37%. 4. The main metabolite in plasma and urine was ASL-8123, which accounted for more than 80% of radioactivity in both samples. This indicates that enzymatic hydrolyses of esmolol in blood and the excretion of ASL-8123 into urine are rapid. Hydroxylated ASL-8123 was detected in minor quantities, but conjugates were not observed in either sample. No sex-related difference was observed in metabolite composition. 5. Radioactivity was completely excreted into feces or urine within a day, and urinary excretion (about 95% of dose) was the major elimination route. After repeated administration (for 7 days), the urine/feces ratio was not different from that after a single administration, and radioactivity was not remained. The biliary excretion of radioactivity was slight (less than 2% of dose) and the transfer into milk was observed, but the elimination of radioactivity in milk correlated with that in maternal plasma.