A series of fertile male mice were rendered temporarily cryptorchid by abdominal fixation of the testes. At intervals the testes were released in pairs of mice and the time noted when subsequent fertility returned. This was found to be about 2 months in most cases but mice with abdominal testes for more than 5 months did not always resume fertility.Similarly neonatal mice had their testes secured in the abdomen and released at intervals. Fertility returned in about 2 months if the testes were released before 2 months after the normal age of puberty. Litters which were produced in both groups were normal.The histological appearance of the testes was similar to that described in the literature.It was concluded that mice previously fertile can resume normal fertility after temporary cryptorchidization for up to 5 months. But mice cryptorchidized from birth only become fertile if the testes are placed in the scrotum before or shortly after puberty.