Antioxidant enzyme activities and glutathione (GSH) content were investigated in the ventricular myocardia of adult (4.5 mo.), mid-age (14.5 mo.), and old (26.5 mo.) male Fischer 344 rats. In addition, the effect of 10 wks of exercise training (T) on these antioxidant systems was evaluated at each age, T was performed on a rodent treadmill for 1 hr/day, 5 days/wk at a speed and grade adjusted for the animal's age. Activities of Superoxide dismutase (SOD), glutathione peroxidase (GPX), and glutathione disulfide (GSSG) reductase were increased by 39%, 27% and 38%, respectively (p<0.05), in old vs young rats. T has no significant effect on antioxidant enzyme activities at any age. Myocardial protein concentration of the aged heart was significantly lower compared to that of the adult or young rats. Total glutathione concentration in the heart was increased in old (12.5±0.6) vs adult (10.7±1.2) and mid-age (9.7±0.9 nmol/mg protein) rats (p<0.05), however the GSH/GSSG ratio was not altered by T in any age group. Myocardial malondialdehyde (MDA) content was increased (p<0.001) with aging. However, MDA content in mid-age and old T rats was decreased by 21 and 25%, respectively, compared to their untrained counterparts (p<0.05). It is concluded that aged hearts are susceptible to oxidative stress despited increased antioxidant capacity and that training may attenuate the age-associated increase in lipid peroxidation in the heart.