Adult male mice of CBA/C57 B1 strain received a whole-body x-irradiation of 400 r. The ultrastructural and cytochemical changes of the spermatogonia and of the Sertoli cells were studied throughout the first 48 hours after this irradiation. Tissues were fixed in osmium tetroxide or glutaraldehyde. The acid phosphatase reaction was carried out using a slightly modified Gomori medium. The first changes observed in spermatogonia were: dilatations of the smooth endoplasmic reticulum, moderate enlargement of the mitochondria and swelling of the nuclear membranes. In a later stage, the injured spermatogonia were phagocytized by the adjoining Sertoli cells, where the complete involution took place. The study of the acid phosphatase localization showed that the numerous dense bodies, appearing in the Sertoli cells in the first hours after irradiation, were heavily loaded with reaction product. They seemed to take an important part in the resorption of the phagocytized spermatogonia. These results were compared with findings of other authors.