In low-renin hypertensive patients, the acute effect of the angiotensin I-converting enzyme inhibitor, captopril, was evaluated in relation to the response of plasma bradykinin (PBK) levels as a parameter of its inhibitory effect on kininase II. Captopril significantly lowered the blood pressure and increased PBK levels. While there was no significant relationship between the reduction of blood pressure and pretreatment plasma renin activity, a significant correlation was observed between the antihypertensive effect of captopril and changes in PBK (r = 0.834, P < 0.01, n = 10). In a patient with primary aldosteronism and in a patient with glucocorticoid responsive hyperaldosteronism, captopril increased plasma PBK with reduction of the blood pressure. In low-renin hypertension, the vasodepressor effect of acute converting enzyme inhibition is due mainly to kinin accumulation rather than inhibition of angiotensin II formation.