Twice-daily captopril (25 mg) and placebo were compared in ten hypertensive patients who were already receiving bendrofluazide. After six weeks therapy, captopril produced significant antihypertensive effects one to six hours after dosing but these did not persist at eleven to twelve hours. Plasma renin concentration was increased for twelve hours after captopril but inhibition of angiotensin II activity was lost by twelve hours. During the period when captopril reduced blood pressure significantly, effective renal plasma flow and hepatic blood flow were unchanged although renal vascular resistance was reduced. There was no evidence that captopril altered plasma sodium, potassium or magnesium concentrations following bendrofluazide. Thus, in thiazide-treated patients, captopril 25 mg produces significant blood pressure reduction for at least six hours after dosing, without impairing renal or hepatic blood flow. However, twice-daily low-dose captopril does not adequately control blood pressure throughout the dosage interval.