T o investigate the mechanism by which angiotensin-converting enzyme (ACE) inhibition attenuates atherogenesis, we have studied the effects of a non-sulfhydryl ACE inhibitor, enalapril, and an angiotensin receptor antagonist, SC-51316, in cholesterol-fed rabbits. After 3 mo of enalapril treatment (10 mg/kg per d, p.o.) the percent plaque areas in the thoracic aortas of treated animals were significantly reduced (controls: 86.8 +/- 3.5%; treated: 31.1 +/- 8%, P < 0.001). Aortic cholesterol content was also reduced (controls: 31.4 +/- 3.2 mg/g tissue; treated: 7.4 +/- 1.8 mg/g, P < 0.001). Enalapril had no significant effect on plasma lipid levels or conscious blood pressure. In a second study, the angiotensin II receptor antagonist SC-51316 was administered at a dose equivalent to enalapril at blocking angiotensin pressor effects in vivo (30 mg/kg per d, p.o.). Evaluation after 3 mo indicated no significant attenuation of aortic atherosclerosis. These results demonstrate that: (a) enalapril attenuates atherogenesis without affecting either blood pressure or plasma lipid levels; (b) antioxidant activity, found with sulfhydryl-containing ACE inhibitors, is not necessary for reducing plaque formation; and (c) the attenuation of atherogenesis by ACE inhibition may not be due to blockade of the renin-angiotensin system. Alternatively, one must consider the multiple effects of ACE inhibition on other hormone systems, such as bradykinin, or the possibility that alternate angiotensin II receptors may be involved in atherosclerosis.