The current study aimed to investigate the effect of endurance exercise training on antioxidant enzyme activities and lipid peroxidation of diabetic rat heart. To this end, thirty male Wistar rats were randomly divided three groups. The control group (C), diabetic group (DC), diabetic-exercise group (DEx). Diabetes was induced by a single intraperitoneal injection of freshly prepared STZ dissolved in 0.05 M of sodium citrate buffer, pH=4.6, (STZ; 45 mg/kg B.wt.). Animals in the exercised groups were exercised on a treadmill 4 days a week for 8 weeks. Catalase (CAT) and glutathione peroxidase (GPx) activities of the heart tissue significantly increased in diabetic rats compared with control animals, whereas superoxide dismutase (SOD) activity was not found significantly different among the experimental groups. On the other hand, endurance training decreased GPx activity in the heart tissue, while CAT activity was not affected by 8-week endurance training. Malondialdehyde (MDA) levels in the heart tissue were significantly increased by 8-week treadmill training. Our results suggest that endurance type exercise may make the heart tissue more susceptible to oxidative stress. Therefore, it may be wise to combine aerobic exercise with other treatment to prevent its adverse effects on antioxidant defense in heart in patients with diabetes mellitus.