GnRH is one of the paracrine/autocrine regulators of hCG secretion produced by the human trophoblast during pregnancy. We hypothesized that GnRH may play a role in the embryonic/endometrial dialogue during early implantation. To examine this hypothesis, we assessed GnRH and GnRH-receptor mRNA and protein expression in human endometrium throughout the menstrual cycle of premenopausal fertile patients. Quantitation of the mRNA was performed by reverse transcription (RT)-competitive polymerase chain reaction (PCR) in the presence of a competitive cDNA fragment. RT-PCR revealed that unfractioned endometrium and isolated endometrial stromal and epithelial cells express GnRH and GnRH-receptor mRNA throughout all phases of the menstrual cycle. Quantitative PCR showed a dynamic pattern in the GnRH mRNA expression throughout the cycle, with a significant increase (p < 0.05) in the secretory phase as compared to the proliferative phase. Furthermore, quantitative competitive PCR of isolated glandular and stromal cells showed higher mRNA levels (p < 0.05) in the luteal phase in both compartments. GnRH immunostaining was localized in all major compartments, with the most intense staining during the luteal phase. On the basis of these data, we suggest that during reproductive life, endometrial GnRH may play a paracrine/autocrine role in the early stages of implantation by modulating embryonic trophoblastic secretion of hCG.