Diabetic patients develop cardiomyopathy characterized mainly by left ventricular contractile dysfunction and congestive heart failure. This study has investigated the effects of diabetes and insulin treatment on lipid peroxidation, vitamin E, and vitamin E-quinone levels in the heart ventricles of rat made diabetic by streptozotocin treatment. Controls were injected with buffer alone; a subgroup of diabetic rats were injected daily with insulin for 2 months. Membrane lipid peroxidation was measured by determining the thiobarbituric acid (TBA)-reactivity. Vitamin E and vitamin E-quinone were measured by using the high pressure liquid chromatography. There was a significant (p < 0.02) increase in the vitamin E-quinone in the heart ventricles of diabetic rats (0.33 +/- 0.05 microgram/mg phospholipid) compared with control rats (0.19 +/- 0.02). This increase was prevented in insulin-treated diabetic rats (0.20 +/- 0.03). Vitamin E levels were higher (14.15 +/- 1.17 micrograms/mg phospholipid) in diabetic rats compared to control rats (9.93 +/- 1.29 (p < 0.03). However, insulin treatment to diabetic rats did not cause any change in vitamin E levels (11.75 +/- 1.02) compared with diabetic rats. TBA reactivity was higher in the heart ventricles of diabetic rats (1.09 +/- 0.11 nmole/mg phospholipid) compared with controls (0.78 +/- 0.08, p < 0.04). Insulin treatment to diabetic rats prevented the increase in the lipid peroxidation (0.79 +/- 0.07); there were no statistically significant differences in TBA-reactivity levels in heart ventricles of insulin-treated diabetic and control rats. This study documents accumulation of vitamin E-quinone and lipid peroxidation products in heart ventricles in diabetic rats, which may have a role in the altered contractile property of the heart ventricles in diabetes.