Angiotensin-converting enzyme (ACE) inhibitors effectively interfere with the renin-angiotensin system and exert various beneficial actions on vascular structure and function beyond their blood pressure-lowering effects. Data from experimental studies showed that angiotensin-converting enzyme inhibitors can attenuate the development of atherosclerosis in a wide range of species. The postulated mechanisms of this atheroprotective effect are the antioxidant actions of angiotensin-converting enzyme inhibitors and their enhancement of the endothelial elaboration of bioactive nitric oxide. The aim of this study was to assess the comparative effects of three angiotensin-converting enzyme inhibitors on endothelial nitric oxide production and action, and on endothelial oxidative stress. Using bovine aortic endothelial cells in culture grown to confluence, we examined the effects of 1, 10, 30 and 60 microM of each of captopril, zofenopril and enalapril on nitrite/nitrate accumulation in the media, cyclic GMP accumulation in the cell lysate, and F(2)-isoprostanes in lipid extracts from the cells. Results showed that the sulfhydryl angiotensin-converting enzyme inhibitor zofenopril has unique properties compared with captopril and enalapril. This compound improves nitric oxide production and bioactivity, and does so in conjunction with decreased endothelial cell oxidant stress. The biochemical basis for this protective mechanism is not entirely clear; however, these actions suggest that zofenopril may reduce endothelial effects of risk factors for atherothrombotic disease.