Changes in generation of reactive oxygen species and antioxidant enzyme activities are associated with differentiation processes. The authors have studied the activity of superoxide dismutase (SOD) in sequentially cut stage-defined segments of rat seminiferous tubules. Great variation was observed in SOD activity along the seminiferous epithelial wave. At its highest, four-fold increases were observed in individual tubules. However, these changes showed no clear correlation to the stages of the cycle. To determine the effect of testosterone withdrawal, rats treated with ethane dimethanesulphonate (EDS) were studied. This treatment had no effect on the pattern of SOD activity along the seminiferous epithelial wave. Testes of other rats were exposed to local 3.0 Gy X-irradiation to cause selective loss of germ-cell populations. SOD activity in the seminiferous epithelium was not affected at 30 min or 7 d after X-irradiation. On day 31 post-irradiation, SOD activity increased at stages XIV-VI, peaking at stage III (P < 0.01 for comparison of stages XIV-VI with the other stages). The data presented here suggest that the activity of SOD in seminiferous epithelium is regulated over a wide range during spermatogenesis. Testosterone plays no major role in the control of seminiferous tubule SOD activity. The loss of spermatocytes and early spermatids by day 31 after X-irradiation revealed a stage-specific increase in SOD activity, which may be associated with the differentiation of elongated spermatids.