The study was designed to evaluate the effects of chronic inhibition of angiotensin-converting enzyme (ACE) on the reactivity of the endothelium and the smooth muscle to vasoconstrictor and vasodilator stimuli in normal rats. Male rats were treated orally for 6 weeks with enalapril (10 mg/kg/day, n = 10) or with placebo (n = 10). Endothelium-dependent relaxations to acetylcholine and adenosine diphosphate were augmented in aortic rings from rats treated with enalapril compared with controls, whereas the response to the endothelium-independent vasodilator SIN-1 were similar. Contractions induced by phenylephrine and angiotensin II remained unchanged. Thus, the results suggest that chronic ACE inhibition enhances the release of relaxing factors from the endothelium in normotensive rats.