L-663,581, 7-chloro-4,5-dihydro-5-methyl-3-(5-(1-methylethyl)-1,2,4-oxadiazol -3-yl)6H-imidazo(1,5-A)(1,4)benzodiazepin-6-one, is an investigational partial agonist for benzodiazepine receptors. This study was designed to characterize the absorption and disposition of the drug and its active metabolites in rats, dogs, and rhesus monkeys. Following intravenous administration (5 mg/kg), L-663,581 was cleared rapidly in all species. The plasma clearance was approximately 95, 40, and 48 ml/min/kg for rats, dogs, and monkeys, respectively. Comparison of urinary recoveries of radioactivity after oral and intravenous dosing indicated that approximately 80-90% of the dose was absorbed in rats and dogs, and 50% in monkeys. The bioavailability was approximately 45% in dogs, 23% in rats, and very low in monkeys after oral administration (5 mg/kg), suggesting an extensive first-pass metabolism in all species. HPLC radiohistograms of urine from all species after intravenous administration revealed that only a trace amount of intact drug was present, indicating that the drug was mainly eliminated by biotransformation. NMR and mass spectral analyses showed that hydroxylation is a major biotransformation pathway. Two active metabolites have been identified as mono- and bis-hydroxy analogs. In addition, the disposition of the monohydroxylated metabolite was also studied in these species. Elimination of the metabolite from plasma was much slower than that of the parent drug in all species. Systemic conversion of L-663,581 to the monohydroxylated metabolite was approximately 43% in rats, 52% in dogs, and only 11% in monkeys.