The preservation and death of germ cells in the neonatal mammalian ovary are linked with the presence of hormones. Estrogens and oxytocin are present at birth in all mammalian vertebrates. The aim of this study was to examine their role in the development of the neonatal ovary and also in the preservation and death of germ cells in the neonatal period: apoptotic phenomena play a fundamental role in the control of their number. Female neonatal mice were treated at birth with estradiol monobenzoate or oxytocin and sacrificed after 5 days. The ovaries were sectioned in toto into semi-thin sections, in order to calculate their volume. Thin sections were also carried out to verify, under the transmission electron microscope (T.E.M.), the cells in apoptosis. The ovaries treated with the greater concentration of estradiol monobenzoate showed a volume that was significantly greater than that of the controls and a reduction of germ cells in apoptosis. The ovaries treated with oxytocin at all degrees of concentration had a volume significantly less than the controls and they also had a higher number of germ cells in apoptosis.