JI, L. L. Antioxidant enzyme response to exercise and aging. Med. Sci. Sports Exerc., Vol. 25, No. 2, pp. 225–231, 1993. Antioxidant enzymes play an important role in defending the cells against free radical-mediated oxidative damage. The present investigations, using rats as models, indicate that antioxidant enzyme systems undergo significant alteration during aging and in response to acute and chronic exercise. Hepatic and myocardial antioxidant enzymes show a general decline at older age, whereas activity of glutathione-related enzymes in the liver and mitochondrial enzymes in the heart increase significantly. Skeletal muscle antioxidant enzymes are uniformly elevated during aging. An acute bout of exercise can increase activity of certain antioxidant enzymes in various tissues. The mechanism for this activation is unclear. Exercise training has little effect on hepatic or myocardial enzyme systems but can cause adaptive responses in skeletal muscle antioxidant enzymes, particularly glutathione peroxidase. These findings suggest that both aging and exercise may impose an oxidative stress to the body.