Abstract High doses of captopril (SQ 14, 225) (120–160 mg/kg/day) were administered orally to normal rats, and the effects on the renin-angiotensin-aldosterone system were observed. Plasma angiotensin converting enzyme (ACE) activity was elevated significantly on the 3rd, 7th and 30th days of captopril administration. ACE activity in the lung and the kidney was significantly decreased on the 1st day then gradually increased, becoming significantly higher than that of controls by the 30th day. Plasma renin activity (PRA) was significantly elevated on the 1st day and remained at a high level until the 30th day. Renal renin content was found to be significantly lower on the 1st and 3rd days. Plasma aldosterone concentration was not affected by captopril treatement, whereas serum potassium concentration was found to be significantly lower on the 1st, 3rd and 30th days. It is suggested that besides its inhibitory action on ACE, captopril has a direct or indirect stimulating action on ACE production as well as on renin release.