The effect of acute angiotensin converting enzyme (ACE) inhibition on the plasma concentrations of atrial natriuretic peptide (ANP) was investigated in a single-blind placebo controlled crossover study in healthy volunteers. 2. Intravenous infusion of 2 mg cilazaprilat resulted in a significant and short lasting inhibition of ACE as evidenced by a decrease of plasma angiotensin II and an increase in plasma renin activity. 3. When compared with placebo cilazaprilat lowered diastolic pressure and increased heart rate significantly. 4. No effect of cilazaprilat was found on plasma ANP levels, suggesting that angiotensin II does not mediate ANP release.