Plasma aldosterone concentration was measured in seven patients before and during long-term angiotensin II suppression with captopril. Plasma aldosterone decreased initially from 62 to 9 pg/ml (p less than 0.01) after 1 month of captopril administration. Thereafter, it began to increase and after 1 year reached a level of 163 pg/ml, which significantly (p less than 0.01) exceeded the pretreatment value. During long-term captopril therapy, plasma renin activity remained elevated and plasma angiotensin II concentration suppressed. The mechanism responsible for the late increase in plasma aldosterone during long-term angiotensin II suppression with captopril remains to be elucidated. Body weight decreased initially, parallel to plasma aldosterone increase, but after 6 months increased again to reach its pretreatment level after 1 year. Nevertheless a sizable and lasting hypotensive effect was observed in all patients.