Recently it was shown that a second GnRH system exists in primates. This study was conducted to investigate whether or not the receptor specific for GnRH type II is expressed in human endometrial and ovarian cancer cells and whether or not GnRH-II has effects on tumor cell proliferation. Expression of GnRH-II receptor mRNA in endometrial and ovarian cancer cell lines was demonstrated using RT-PCR and Southern blot analysis. The proliferation of these cell lines was dose- and time-dependently reduced by native GnRH-II. These effects were significantly more potent than the anitproliferative effects of equimolar doses of GnRH-I agonist Triptorelin (p<0.001). In the GnRH-II receptor positive but GnRH-I receptor negative ovarian cancer cell line SK-OV-3 native GnRH-II but not GnRH-I agonist Triptorelin had antiproliferative effects.