The aim of this study was to investigate the effect of aerobic exercise training on activities and mRNA levels of catalase (CAT), glutathione peroxidase (GPX), Cu,Zn- and Mn-superoxide dismutases (SOD), TBARS content, and xanthine oxidase (XO) activity, in soleus muscle from young and aged rats. The antioxidant enzyme activities and mRNA levels were markedly increased in soleus muscle with aging. TBARS content of soleus muscle from the aged group was 8.3-fold higher as compared with that of young rats. In young rats, exercise training induced an increase of all antioxidant enzyme activities, except for Cu,Zn-SOD. XO also did not change. The TBARS content was also increased (2.9-fold) due to exercise training in soleus muscle from young rats. In aged rats, the activities of CAT, GPX and Cu,Zn-SOD in the soleus muscle did not change with the exercise training, whereas the activities of Mn-SOD (40%) and XO (27%) were decreased. The mRNA levels of Mn-SOD and CAT were decreased by 42% and 24%, respectively, in the trained group. Exercise training induced a significant decrease of TBARS content (81%) in the soleus muscle from aged rats. These findings support the proposition that exercise training presents an antioxidant stress effect on skeletal muscle from both young and aged rats.