Oxidative stress (OS) induced by acute exercise is reduced by chronic exercise. Ozone (O(3)) exposure produces OS. The aim of this study was to determine if aerobic exercise (AE) reduced OS produced by O(3). A pilot experiment was performed with male Wistar rats submitted to AE (trained to swim 90 min/day). Adaptation to exercise was demonstrated three weeks after training by means of changes in reduced nitrates (NO(x)) in plasma. Therefore, two-week training was chosen for the following experiments. Six of twelve trained rats were exposed to O(3) (0.5 ppm, 4 h/day, one hour before exercise). Two groups of sedentary animals (n = 6 each) were used as controls, one of which was exposed to O(3). At the end of the experiments NO(x), 8-isoprostane (8-IP), malondialdehyde (MDA), superoxide dismutase (SOD) activity, and carbonyls (CBs) were measured in plasma. CBs did not change in any group. O(3)-induced OS was manifested by reduced NO(x) and SOD activity, as well as increased 8-IP and MDA. Exercise significantly blocked O(3) effects although SOD was also decreased by exercise (a greater drop occurring in the O(3) group). It is concluded that AE protects against OS produced by O(3) and the effect is independent of SOD.