Apoptosis has been found to occur during regression of the corpus luteum (CL) in many species. The Fas (APO-1/CD95) receptor, a transmembrane protein that induces apoptosis in the cell when bound to Fas ligand (FasL), may be involved. This study established and quantitated the presence and regulation of Fas receptor and FasL in the rat CL during pregnancy and postpartum. Using immunohistochemistry, FasL was localized in CL during pregnancy and postpartum. Fas was localized at Day 1 of pregnancy and at the time of luteolysis. Both Fas and FasL mRNA were found to be expressed throughout pregnancy and postpartum using reverse transcription-polymerase chain reaction (RT-PCR). Relative quantitative RT-PCR established that expression of FasL mRNA increased significantly at Day 22 of pregnancy and decreased by Day 3 postpartum. Spontaneous apoptosis of rat CL placed in an in vitro culture model with serum-free medium was examined by analysis of extracted DNA using 3' end-labeling. Treatment with an anti-rat Fas monoclonal antibody demonstrated a reduction in the occurrence of spontaneous apoptosis. These data support a role for Fas receptor and FasL in rat CL apoptosis during luteolysis.