This study was conducted to investigate a possible systemic effect of angiotensin-converting enzyme inhibitors (ACEi) on tissue antioxidant defenses. CF1 mice (4-mo-old females) were administered either water (control) or water containing enalapril (20 mg/l) or captopril (50 mg/l) during 11 wk. Neither enalapril nor captopril treatment had an effect on body mass or brain, kidney, or heart weight relative to controls. CuZn-superoxide dismutase (SOD) activity was increased by enalapril treatment in kidney medulla (27%), heart (24%), and erythrocytes (19%) and by captopril treatment in kidney medulla (43%) and heart (54%) relative to controls. Mn-SOD and catalase activities were unaffected by either treatment. Enalapril, but not captopril treatment, increased Se-glutathione peroxidase activity in renal medulla (19%). Nonenzymatic antioxidant defenses, evaluated by tert-butyl hydroperoxide-initiated chemiluminescence (HICL), were enhanced in kidney cortex (48%) by enalapril and in brain by enalapril (44%) or captopril (36%) treatment relative to controls. As evaluated in vitro by HICL and thiobarbituric acid-reactive substances formation, captopril had a free radical scavenger activity, whereas neither enalapril nor lisinopril was effective. These results suggest that ACEi may protect tissues from oxidative damage by increasing enzymatic and nonenzymatic antioxidant defenses.