We have determined the effects of maximal and submaximal cycloergometer tests on the antioxidant enzyme defences of neutrophils and lymphocytes. We also compared the neutrophil and lymphocyte basal enzyme antioxidant activities. A total of 17 well-trained amateur athletes, runners, and cyclists participated in this study. Two tests were performed on an electromagnetic reduction cycloergometer: the maximal exercise test, and the submaximal prolonged exercise test. Blood samples were taken before and after the tests. Basal enzyme activity of superoxide dismutase was higher in lymphocytes but neutrophils presented higher activities of catalase and glutathione peroxidase. The maximal test increased the circulating number of lymphocytes and the activities of catalase and glutathione peroxidase. No changes were observed in lymphocyte number or in lymphocyte antioxidant enzyme activities after the submaximal test. The circulating number of neutrophils increased significantly after the submaximal test. Maximal and submaximal tests decreased the activities of neutrophil glutathione dependent antioxidant enzymes (glutathione peroxidase and glutathione reductase), but no changes were observed in catalase or superoxide dismutase activities after either test. Neither the maximal nor submaximal test produced increases in serum activities of lactate dehydrogenase and creatine kinase (CK).