Plasma concentrations and urinary excretion rate of vasopressin (VP) were examined in ten cases of severe hypertension before and during short-term treatment by Captopril (SQ 14225). Before Captopril, plasma and urinary VP were high (respectively 5.24 pmol/l and 68 pmol/day) and positively correlated to plasma renin activity (PRA) and plasma aldosterone (PA). The decline in blood pressure (mean -15%) after Captopril was correlated not only to initial PRA and PA values, but also to plasma (r = 0.89; P less than 0.001) and urinary (r = 0.78; P less than 0.01) VP values. The initial dose of Captopril (1 mg/kg) induced a rapid decrease in blood pressure whereas plasma VP did not rise and aldosterone decreased. At the eighth day of Captopril treatment (mean daily dose 6 +/- 1.5 mg/kg) the drop in blood pressure (-12%) and in aldosterone persisted together with a significant reduction in plasma (1.18 pmol/l; P less than 0.01) and urinary (25 pmol/day; P less than 0.01) VP. It is suggested that these sustained simultaneous reductions in the rates of secretion of vasopressin and aldosterone are both elements of the antihypertensive effect of Captopril.