The erythrocyte antioxidative defense system was investigated before and after a 12km run. The effect of exercise with a bicycle ergometer on this system was also examined one and seven days after the. 12km run. Five untrained healthy men, aged 18-21 years, participated in the present study. Plasma creatine kinase (CK) activity increased after running and reached a maximum on the next day (p<0.005) . Levels of the reduced form of glutathione (GSH) decreased by 24.6% after running (p<0.005), but by 7 days later the values recovered. Levels of lipid peroxides (LPO, expressed as thiobarbituric acid-reactive substances) fell slightly after running (13.4%, not significant), then showed a significant decrease on the next day (34.7%, p<0.005) and recovered by the seventh day. Neither glutathione peroxidase (GPX) nor glutathione reductase (GR) activities (with or without FAD) showed significant changes after the 12km run. Activities of catalase (CAT) were lowest after the exercise with a bicycle ergometer on the following day (p<0.05 vs. initial levels) . Levels of GSH, GPX, GR and LPO did not change after this exercise. The decreased level of GSH after the 12km run indicates the presence of oxidative stress, while moderate exercise with a bicycle ergometer does not affect the erythrocyte glutathione metabolism in untrained men. It is also suggested that catalase activity decreases after the repeated exercise.