Summary. The effect of whole-body irradiation on the fertility of the male rat and mouse has been examined by systematic weekly matings of the treated animals. In both species, irradiation with 300r produces a short period of sterility associated with oligospermia. This occurs at about 45 days after treatment in the mouse and 65 days in the rat. Histological examination shows that the main effect is on the spermatogonial stages but there are obvious differences in the timing and extent of these radiation effects in the two species. The time from treatment to the onset of sterility is a measure of the overall duration of spermatogenesis. The highest dose level used, 500r, did not cause a rapid onset of sterility in contrast to the action of some alkylating agents, which rapidly induce infertility.