Recent studies have suggested that the calcium antagonists have an antiatherogenic antioxidant property. The effects of the calcium antagonists on reactive oxygen species (ROS)-related enzymes, however, remain unknown. We hypothesized that the calcium antagonists inhibit oxidative stress in the hearts of stroke-prone spontaneously hypertensive rats (SHRSP) through the ROS-scavenging enzymes known as superoxide dismutases (SODs). Male 12-week-old Wister-Kyoto rats (WKY) and SHRSP were used for the study. SHRSP were randomized and treated for 6 weeks with a vehicle, amlodipine (5 mg/kg/day), or enalapril (10 mg/kg/day). NAD(P)H oxidase activity was measured by a luminescence assay, and SOD activity was measured spectrophotometrically. Protein expressions were analyzed by immunoblots. Both drugs showed equipotent effects on systolic blood pressure, left ventricular hypertrophy and fibrosis, the wall-to-lumen ratio, the manganese SOD activity, ROS, and the endothelial NO synthase expression in the SHRSP hearts. Furthermore, amlodipine significantly restored copper/zinc-containing SOD (Cu/ZnSOD) expression and its activity in SHRSP hearts to a level equal to that of WKY more effectively than did enalapril (p <0.05), whereas enalapril downregulated NAD(P)H oxidase activity more than did amlodipine (p <0.05) in the SHRSP hearts. Furthermore, amlodipine restored Cu/ZnSOD expression and its activity in SHRSP hearts to a level equal to that in WKY hearts, and this restoration was significantly more effective than that by enalapril (p <0.05); on the other hand, enalapril induced a greater downregulation of NAD(P)H oxidase activity in SHRSP hearts than did amlodipine (p <0.05). Thus, amlodipine may inhibit vascular remodeling and oxidative stress in the SHRSP heart by efficiently upregulating Cu/ZnSOD, suggesting that the calcium antagonist may exhibit an antiatherogenic antioxidative action beyond blood-pressure lowering through the restoration of Cu/ZnSOD activity in the heart in cases of hypertension. (Hypertens Res 2004; 27: 877-885)