Apoptosis of granulosa cells is the cellular mechanism of ovarian follicular atresia, and cytokines have been implicated as potential atretogenic factors. We therefore investigated the possible role of the cytokine Fas ligand (FasL) and its receptor Fas in apoptosis during ovarian follicular atresia induced by gonadotropin withdrawal. Immature rats pretreated with eCG were injected 24 h later with an antiserum generated against eCG (antibody group) or preimmune rabbit serum (control), and ovaries were removed 1 and 24 h after treatment. The eCG antiserum caused a dose-dependent inhibition of the significant increase in ovarian weight observed between 24 and 48 h after eCG treatment. In situ detection of fragmented DNA in histological sections identified cell death in atretic but not healthy small and medium-sized antral follicles of the antibody group. Cell death was distributed in a scattered pattern throughout the granulosa cell layer of small atretic follicles but was localized primarily in granulosa cells lining the antral cavity of atretic medium antral follicles. Immunohistochemistry of adjacent histological sections revealed intense positive immunostaining for Fas and FasL in granulosa cells of atretic small and medium antral follicles in a pattern coincidental to the localization of cell death. Intense FasL staining was evident in the theca cells of healthy small antral follicles. An increase in low molecular weight DNA (DNA "ladders") indicative of apoptosis was evident in granulosa cells of the antibody group. Western analysis demonstrated increased levels of both Fas and FasL in the granulosa cells of the antibody group. These results demonstrate that both Fas and FasL are present in ovarian granulosa cells and that FasL may be the signal that induces granulosa cell apoptosis during atresia at the penultimate stage of ovarian follicular development.