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
