To investigate the mechanism of germ cell death in postnatal stage of mouse, the involvement of apoptotic executioners, caspase-3 and caspase-activated DNase(CAD), and apoptotic initiators, Bax, Fas and Fas ligand, in the germ cell death has been studied. Immuno-labels of active caspase-3 and CAD were located in TUNEL-positive, apoptotic, oocytes as well as normal oocytes of primary or secondary follicles. CAD immuno-labels were also detected in the nucleus of TUNEL-positive oocytes. Most of oocytes showing positive immuno-labeling of active caspase-3 or CAD had vacuoles in their cytoplasm, which is the morphological characteristic of oocyte during follicular atresia. Bax immuno-stains were detected in the atretic oocytes which showed the vacuole in their cytoplasm. Positive immuno-labels for Fas ligand was localized in TUNEL-positive or atretic oocytes. Presence of immunoreactivity of active caspase-3 and CAD in TUNEL-positive germ cells implicate that active caspase-3 and CAD might play a role in germ cell apoptosis during early development of mouse ovarian follicle. Immunohistochemical localization of Bax and Fas ligand in TUNEL-positive oocytes suggests that these might be the most plausible modulator of oocyte apoptosis.