Physical activity (PA) is associated with a reduced risk of coronary heart disease, and may favorably modify the antioxidant-prooxidant balance. This study assessed the effects of aerobic PA training on antioxidant enzyme activity, oxidized LDL concentration, and LDL resistance to oxidation, as well as the effect of acute PA on antioxidant enzyme activity before and after the training period. Seventeen sedentary healthy young men and women were recruited for 16 weeks of training. The activity of superoxide dismutase in erythrocytes (E-SOD), glutathione peroxidase in whole blood (GSH-Px), and glutathione reductase in plasma (P-GR), and the oxidized LDL concentration and LDL composition, diameter, and resistance to oxidation were determined before and after training. Shortly before and after this training period they also performed a bout of aerobic PA for 30 min. The antioxidant enzyme activity was also determined at 0 min, 30 min, 60 min, 120 min, and 24 h after both bouts of PA. Training induces an increase in GSH-Px (27.7%), P-GR (17.6%), and LDL resistance to oxidation, and a decrease in oxidized LDL (-15.9%). After the bout of PA, an increase in E-SOD and GSH-Px was observed at 0 min, with a posterior decrease in enzyme activity until 30-60 min, and a tendency to recover the basal values at 120 min and 24 h. Training did not modify this global response pattern. Regular PA increases endogenous antioxidant activity and LDL resistance to oxidation, and decreases oxidized LDL concentration; 30 min of aerobic PA decreases P-GR and B-GSH-Px activity in the first 30-60 min with a posterior recovery.