THis study was designed to investigate the mechanisms of captopril's chronic effect on arterial pressure and renal function. In dogs maintained on high sodium intake (250 mEq/day), 6 days of captopril infusion caused no change in arterial pressure, renal hemodynamics, sodium excretion or plasma aldosterone concentration. Infusion of captopril for 7 days also caused no significant changes in arterial pressure or renal function in dogs made hypertensive by chronic infusion of angiotensin II and high sodium intake, a model of hypertension in which plasma renin activity is undetectable and prostaglandin and bradykinin formation may be elevated. In dogs maintained on low sodium intake, chronic infusion of captopril decreased arterial pressure and plasma aldosterone concentration markedly while increasing effective renal plasma flow. Infusion of aldosterone (200 micrograms/day) for 8 days during captopril infusion restored plasma aldosterone concentration but did not significantly change arterial pressure or renal function, indicating that decreased plasma aldosterone concentration did not play a major role in the hypertensive and renal effects of captopril. However, angiotensin II infusion (10 ng/kg/min) for 8 days during captopril infusion restored arterial pressure, plasma aldosterone concentration and renal function toward control levels. These data suggest that the effects of captopril on arterial pressure, renal hemodynamics and electrolyte excretion are mediated primarily by decreased angiotensin II formation.