1. We have evaluated the effects of the angiotensin-converting enzyme inhibitor enalapril on renal function and oxidative status in the kidney of Otsuka Long-Evans Tokushima Fatty (OLETF) rats, an animal model of spontaneous onset of type 2 diabetes mellitus. 2. Enalapril (5 mg/kg) or vehicle (distilled water) was given once daily by gavage to 22-week-old male OLETF rats for 32 weeks. Long-Evans Tokushima Otsuka (LETO) rats, the control animals for OLETF rats, received vehicle alone (n = 10 in each group). 3. Enalapril attenuated the rise in blood pressure mildly, but significantly. Enalapril significantly blunted the development of proteinuria without a significant effect on creatinine clearance. At the end of the study period, the lipid peroxide content in the renal cortex was significantly increased in OLETF compared with LETO rats, in which enalapril had no effect on lipid peroxide content. Enalapril enhanced the activity of catalase in the renal cortex of OLETF rats, but had no effect on the activity of either superoxide dismutase or glutathione peroxidase. 4. These results suggest that oxidative stress may be involved in the development of nephropathy in type 2 diabetes. Enalapril exhibited renoprotective effects without changing lipid peroxides in the kidney, suggesting that the beneficial effects of the compound on diabetic renal damage in OLETF rats may not be mediated through an anti-oxidative action.