14 healthy subjects (8 males and 6 females), aged 25-40 years, were studied before and after oral administration of a single dose of 50 mg captopril as well as after 3 d treatment with 100 mg captopril daily per os. We found that in addition to the well-known effects on the renin-angiotensin-aldosterone system, captopril, after 3 d treatment, significantly increases plasma and urinary kallikrein activity, plasma vasopressin and urinary prostaglandin (PG) E2. Atrial natriuretic peptide did not change significantly after either the single dose or the short-term treatment. We conclude that the blood pressure lowering effect of captopril could be mediated by increasing activity of the kallikrein-kinin system and of PGE2 without any participation of atrial natriuretic peptide.