Captopril, an angiotensin-converting enzyme inhibitor with antihypertensive properties, was given by mouth and intravenously in 10-mg doses to five healthy subjects. After intravenous dosing, semilogarithmic plots of captopril blood levels : time showed a triexponential decay. Data were analyzed using an open three-compartment model. The average volume of distribution (Vd) was 0.2 l/kg for the central compartment and 2 l/kg for the elimination (beta) phase. The Vd at steady-state was 0.7 l/kg. The total body clearance of captopril averaged 0.8 l/kg/hr and the mean blood half-life during the beta phase was 1.9 hr. In the 0- to 96-hr urine, after intravenous and oral drug, excretion of radioactivity accounted for 87% and 61% of dose. In the 0- to 24-hr urine, averages of 38% (intravenous) and 24% (oral) of the doses were excreted as unchanged captopril. Absolute absorption of the radioactive oral dose was 71% and the absolute oral bioavailability of captopril was 62%.