Because elevated oxidative stress may exacerbate cardiovascular complications of diabetes mellitus, the current study aimed to investigate the effects of treatment with either vitamin A, an antioxidant, or with insulin on lipid peroxidation products and antioxidant enzyme activities of diabetic rat heart. Also to evaluate whether a combination of vitamin A and insulin exerts more beneficial effects than treatment with each agent alone. Rats were made diabetic with a single injection of streptozotocin (STZ, 55 mg kg−1 i.p.). Two days after STZ-injection, one group of diabetic rats was treated with vitamin A (retinol acetate, 30 mg kg−1 day−1 i.o.) for 12 weeks. A second group of diabetic rats was untreated for 6 weeks and then treated for another 6 weeks with insulin (8–10 IU rat−1 day−1 s.c.). Both therapies were applied to another group of diabetic rats for assessment of combined therapy with vitamin A plus insulin. Hearts from 12-week untreated diabetic animals showed about a four-fold increase in the level of thiobarbituric acid reactive substances (TBARS), indicative of increased lipid peroxidation. This was accompanied by approximately 100% increase in both catalase and glutathione peroxidase (GSHPx) enzyme activities. Therapy with insulin alone caused a small but significant improvement in plasma TBARS as well as GSHPx activities, but no significant change in plasma catalase in diabetic animals. Diabetes-induced disturbance in TBARS was almost completely prevented by vitamin A therapy. Although, a similar degree of activities for GSHPx was determined in diabetic animals treated with each agent alone, combination therapy was found to be more effective than single therapies in the recovery of GSHPx of diabetic heart. In contrast to insulin single therapy, vitamin A alone significantly prevented an increase in catalase activity of diabetic heart, and a combination of these agents did not supply any further benefit. Superoxide dismutase (SOD) activity was not found significantly different among the experimental groups. STZ-diabetes also resulted in less plasma retinol and retinol-binding protein (RBP), which was significantly improved by insulin single therapy while vitamin A used alone, failed to increase plasma retinol and RBP levels of diabetic animals. Our findings suggest that single therapy with insulin is unable to preclude oxidative reactions in diabetic heart to the same extent as obtained by vitamin A therapy alone, in spite of allowing recovery of normal growth rate and improved vitamin A metabolism in diabetic rats. A combination of insulin with vitamin A may provide more benefits than use of either agent alone in the treatment of general characteristics of diabetes and the maintenance of antioxidant defence of diabetic heart and thus in the reduction of peroxidative stress-induced cardiac injury. Copyright © 2001 John Wiley & Sons, Ltd.