Corresponding to the expression of Fas in the ovarian oocytes as previously reported (Guo et al., Biochem Biophys Res Commun 1994; 203:1438–1446; Mori et al., JSIR 1995; 9:49–50), the expression of Fas ligand (FasL) in the ovarian follicle was found to be restricted in the area of granulosa cells by the indirect immunofluorescence (IIF) test. Reverse transcriptase/polymerase chain reaction (RT/PCR) technique coupled with Southern blot hybridization analysis showed that the highest level of FasL mRNA was demonstrated in murine ovaries and granulosa cells 1 day after the administration of pregnant mare's serum gonadotropin (PMSG), while the level of FasL mRNA became very weak on the day 5, respectively. The observed gradual decrease in FasL mRNA could not be attributed to a generalized degradation of cellular RNA during atresia, as evidenced by the presence of constitutive expression of elongation factor 1α (EF-1α) mRNA in murine ovaries and granulosa cells treated with PMSG. Furthermore, in situ hybridization analysis with a FasL-specific probe confirmed that FasL was specifically localized in the granulosa cells of most follicles and its expression was regulated by PMSG administration. FasL localized in granulosa cells might possibly play an important role in the formation of the ovarian atretic follicles, most likely depending on PMSG administration.