Exercise induces free radical formation in muscle and liver, and oxidative damage, such as lipid peroxidation. The amount of damage depends on exercise intensity, training state and the tissue examined and can be reduced through dietary supplementation of antioxidants such as vitamin E and possibly coenzyme Q10. Supplementation with antioxidants does not increase maximal aerobic capacity or maximal exercise capacity; effects on endurance capacity are unclear. Deficiency of vitamin E or vitamin C greatly reduces endurance capacity, whereas selenium deficiency has no effect on endurance capacity. In studies by the authors, urinary output of the oxidatively damaged RNA base 8-hydroxyguanosine was not affected by several submaximal exercise bouts nor by supplementation with vitamins E and C and beta-carotene in moderately trained humans. In rats, endurance training caused an increase in oxidative damage, as measured by the protein carbonyl concentration of muscle, but not liver. Muscle protein carbonyl concentration returned to normal on detraining. These results indicate that the search for oxidative damage due to exercise and the effects of antioxidant manipulation on such damage should ideally involve examination of several indices of oxidative damage in various tissues after exercise and training.