The pharmacokinetics, safety, and pharmacologic effects of OPC-21268, a nonpeptide orally active vasopressin V1 receptor antagonist, have been investigated in 33 healthy subjects. First, 24 subjects were randomly divided into 3 groups of 8, 6 of whom were given 2 ascending single oral doses out of 6 (10, 50, 150, 300, 450, and 600 mg) of OPC-21268 after an overnight fast. The remaining two subjects in each group received placebo as control at each dosing. Additionally, after this procedure, the 6 subjects who received 50-mg single doses were given the same dose in a nonfasting condition. After the single-dose study was completed and the safety and tolerability were ascertained, the remaining 9 subjects, including 3 controls, were given 300 mg of the drug 3 times daily for 7 days (days 3-9) and were given single 100-mg oral doses before (day 1) and after (day 10) this repeated-dose study. OPC-21268 plasma concentrations declined in a monoexponential or biexponential pattern after reaching the maximum plasma concentrations (Cmax). The mean (+/- standard error of the mean) plasma half-life (t1/2) of the alpha phase ranged from 1.31 +/- 0.11 to 1.78 +/- 0.15 hours, and the mean t1/2 of the beta phase ranged from 4.31 +/- 0.28 to 6.28 +/- 0.59 hours. The area under the concentration (AUC0-infinity) and Cmax were proportional to the dose (P < .001).(ABSTRACT TRUNCATED AT 250 WORDS)