Captopril tablets of two different producers were tested for bioequivalence as well as therapeutic equivalence. The pharmacokinetics, the time course of plasma angiotensin-converting enzyme inhibition, and the changes of systolic and diastolic blood pressure after administration of drugs were studied. In a balanced, randomized two-way crossover design, two single doses of 50mg each of captopril were administered orally to twelve male volunteers. Peak blood levels of free captopril were observed about 0.85 hour after the dose, and practically free captopril could not be detected in blood within 8 hours. Peak free captopril levels of both compounds were almost identical(Capoten®, 464.3ng/ml ; Capril®, 504.6ng/ml). No statistically significant difference was identified between two compounds when area und the concentration time curve, peak level, time to peak were compared. Inhibition of plasma angiotensin-converting enzyme to blood free captopril concentration showed the hyperbolic concentration-response relationship with IC₅₀ value of 7.4ng/ml. The area under the percent angiotensin-converting enzyme inhibition versus time curve were quite similar after administration of both drugs. The compounds were also found to be equivalent on the premise that no significant difference was detected when the time courses of systolic and diastolic blood pressure reduction were compared.