Fas ligand induces cell death by means of apoptosis in a variety of cell types when cross-linked with its natural receptor, Fas. GnRH receptor-bearing tumors undergo apoptosis in vivo and in vitro with GnRH agonists. To provide a potential association of the Fas system with the antiproliferative signaling process of GnRH receptor, we have evaluated the regulation of Fas ligand expression in GnRH receptor-positive tumors and cloned cell lines known to have substantial GnRH receptors. Surgically removed uterine endometrial carcinomas and ovarian carcinomas had been screened for GnRH receptor expression before analysis. Fas ligand protein was characterized by immunoblotting of membrane proteins with the specific antibody. Fas ligand messenger RNA was determined by RT-PCR using oligonucleotide primers synthesized according to the published Fas ligand sequence. Incubation with a GnRH analog (1 mumol/L) induced the expression of Fas ligand messenger RNA and immuno-reactive Fas ligand with a lag time of 48 h in cloned cell lines (endometrial carcinoma HHUA cells, and ovarian carcinoma SK-OV-3 and Caov-3 cells). There was no detectable Fas ligand expression within 24 h. The stimulatory effect of GnRH on Fas ligand protein expression revealed a dose dependency; a half-maximal effect occurred with 10 nmol/L GnRH analog (P < 0.01). The stimulated Fas ligand expression could be neutralized by displacement of GnRH from its receptor by GnRH antagonist antide. Cells isolated from GnRH receptor-bearing ovarian carcinomas and uterine endometrial carcinomas gave identical results to those obtained in cloned cell lines. These data demonstrate the functional coupling of stimulated Fas ligand expression to GnRH receptor activation. Increased Fas ligand level within the GnRH receptor-bearing tumors might promote apoptotic cell death through attack on intratumoral Fas-positive cells that could, at least in part, account for the antiproliferative action of the hormone.