This study investigated the onset of age-related changes in the myocardial antioxidant enzymes and apoptosis and the vulnerability of the myocardium to oxidative stress following exercise training. Few studies have investigated the influence of the most prevalent life-prolonging strategy physical exercise, on the age increment alterations in the myocardial antioxidant enzymes and apoptosis at mid age and to determine whether exercise-induced antioxidant defense system could attenuate lipid peroxidation. Thirty six male Wistar rats were randomly assigned to exercise trained (n = 18) and sedentary (n = 18) groups. The rats in the training group went under 12, 24 and 36 weeks of moderate exercise trainings (25 m·min^-1 for 60-min with a 0% slope). Six sedentary controls were killed together with each exercise group at the end of the training programs. Levels of thiobarbituric acid-reactive substances (TBARS) and catalase (CAT) activity in myocardial homogenates were unchanged by training irrespective of the protocol duration. However, an increased content of the TBARS was detected in hearts from both the 24 and 36-week trained and sedentary control rats when compared with their corresponding 12-week groups (p<0.01). The activity of superoxide dismutase (SOD) remained unchanged after the 12-week training period whereas a significant increase was observed in heart homogenates of 24-week trained animals as compared with their sedentary controls (p<0.05). The activity of glutathione peroxidase (GPX) remained unchanged. The rates of apoptosis which was detected by ELISA assays, were significantly modified after 24 and 36-week of training (p<0.05). These results demonstrate that a long-term endurance training (24 weeks) induced increases in SOD activities in rat myocardium and elicited a marked reduction in apoptosis rate. However, a shorter training program (12 weeks) was not effective in increasing heart antioxidant defenses.