To examine the role of free radicals in diabetic cardiomyopathy, myocardial antioxidants as well as lipid peroxide content were examined in rats made diabetic with a single injection of streptozotocin (65 mg/kg i.v). At 4 wk, the left ventricular peak systolic (LVSP) as well as aortic pressures were depressed in the diabetic group. Hearts from diabetic animals showed about a 100% increase in thiobarbituric acid reactive substances (TBARS), indicating increased lipid peroxidation. This was accompanied by about a 50% decrease in superoxide dismutase (SOD) and 60% decrease in glutathione peroxidase (GSHPx) enzyme activities. Catalase activity in these hearts showed a small but significant increase. Treatment with probucol (10 mg/kg i.p., on alternate days), a known lipid-lowering drug with strong antioxidant properties, was initiated 1 d after the induction of diabetes and was continued for 4 wk. In probucol-treated diabetic animals, LVSP was not different from controls. Probucol treatment caused a small but significant improvement in serum insulin and decrease in glucose levels as well as increased myocardial SOD, GSHPx, and catalase activities with a concomitant decrease in TBARS in the diabetic animals. These data provide evidence that diabetic cardiomyopathy is associated with an antioxidant deficit, and a better cardiac function due to treatment with probucol may be related to the improved insulin levels as well as maintenance of the antioxidant status of the heart.