During the first days of postnatal life in rats the male germ cells (gonocytes) proliferate and move towards the seminiferous tubule basal lamina maturing into spermatogonia. This process is necessary for spermatogenesis and can be affected by estrogen (E); therefore, it is important to determine whether the damaging mechanism induced by E administration during the postnatal period impairs gonocyte maturation. One-day-old rat pups were given 1 microg 17-beta-estradiol daily and studied at 3, 5, 8, 10 and 16 days of age, corresponding to the critical gonocyte differentiation period in the rat. Testicles were isolated and the number of gonocytes in contact with the basal lamina of the seminiferous tubule was estimated, as well as the proliferation rate and apoptosis of the gonocytes. We observed that the administration of E changed the migration of gonocytes towards the basal lamina, decreased cell proliferation and increased apoptosis, resulting in a decrease in spermatogonia and spermatocytes. The migration of gonocytes and subsequent proliferation is required for survival of this germ cell type. The lack of maturation and the death of gonocytes could be one of the causes of infertility following exogenous E treatment.