The present work has used controlled conditions to study how physical activity and stress affect oxidant and anti-oxidant systems in a human model. Stress test consisting of one hour exercise at a cycloergometer with intensity over the 75% of the maximal cardiac frequency was followed by non-significant changes of malonyldialdehyde, assayed as a marker of lipid peroxidation, and by an increase of erythrocyte catalase and plasma and erythrocyte glutathione peroxidase after the test. These findings suggest that antioxidant response is broadly versatile and adaptable, and that physical activity may prevent extended cellular damage with consequent flogosis. Hence, controlled physical exercise may contribute to protect target organs, including cardiovascular systems, against oxidative stress.