OBJECTIVE: The present study evaluated the role of experimental oxidative stress (induced by feeding diets with different concentrations of selenium [Se], a trace nutrient and potent antioxidant) on male reproductive activity in mice. METHODS: To create different levels of oxidative stress in male mice, three diets with different levels of Se were fed to different groups for 8 wk. Mice in group 1 were fed a yeast-based diet, which is considered a Se-deficient diet (0.02 ppm). Mice in groups 2 and 3 were fed with an Se-deficient diet supplemented with 0.2 and 1 ppm Se as sodium selenite, respectively. RESULTS: After completion of the feeding schedule, a significant decrease in Se levels were observed in Se-deficient mice (group 1), whereas Se levels greatly increased in the Se-excess mice (group 3). Glutathione peroxidase activity was greatly decreased in the liver and testis in group 1, whereas glutathione-S-transferase activity was significantly increased in the testis. No significant change was found in activities of glutathione peroxidase and glutathione-S-transferase in group 3 compared with group 2. Cell kinetics showed a significant decrease in the number of pachytene spermatocytes and young and mature spermatids in group 1 compared with group 2. No appreciable change was observed in the germinal cell population in group 3. A significant decrease in sperm number was observed in group 1 compared with group 2. No change in these parameters was observed in group 3. The fertility status of mice in terms of percent fertility and litter size also exhibited a significant decrease in the reproductive ability of group 1. No change in these parameters was observed in group 3 compared with group 2. CONCLUSION: The present results clearly demonstrate the effect of oxidative stress generated by feeding different concentrations of Se on cell kinetics in the testis and, hence, its effect on the reproductive ability of male mice.