The time course of various doses of captopril (C) alone, hydrochlorothiazide (H) alone, and C with H was followed in 142 subjects with mild hypertension. Doses of C were 12.5, 25, and 50 mg three times daily or 37.5 mg twice daily and dose of H was 25 mg twice daily. Time-course measurements were made during continuous treatment, with morning dose omitted on the test day. After C alone, diastolic blood pressure (BP) fell to its lowest point 2 to 5 hr and then gradually returned toward baseline. Reductions from levels before treatment were present 13 hr after a dose and there were no significant differences between various doses of C at that time. Systolic BP fell only with the larger doses. When C was combined with H, both systolic and diastolic BP decreased more than with either C or H alone, reaching a nadir at 2 hr. BP remained significantly below levels before treatment for at least 13 hr after dosing. Except for the response to the lowest (12.5 mg) dose, small doses of C alone had the same effect as large doses on both degree and duration of antihypertensive response. Effect was considerably enhanced by addition of a thiazide diuretic.