Neuropeptides such as gonadotrophin releasing hormone (GnRH) are presumed to play an important role in the regulation of the function and growth of human placenta. Knowledge about the placental site of GnRH expression and the eventual co-localization of its peptide with the GnRH receptor (GnRH-R) is crucial for a better understanding of possible autocrine/paracrine mechanisms. We therefore investigated these questions by use of in-situ reverse transcription-polymerase chain reaction (RT-PCR) alone or in combination with immunocytochemistry in human first and third trimester placentae. Paraffin-embedded placental sections (7 microm thick), or single trophoblasts in monolayer cultures for up to 3 days, were treated with proteinase K. Following RT with GnRH or GnRH-R specific oligoprimers, PCR was performed employing primers with exon-exon overlaps to exclude non-specific DNA amplification. Detection of the amplicons was accomplished by nested PCR which was performed with digoxigenin-labelled dUTP and nitroblue tetrazolium/5-bromo-4-chloro-3-indoyl-phosphate (NBT/BCIP) for substrate visualization. The GnRH peptide was detected using a sandwich-antibody assay. GnRH and GnRH-R gene expression was found in all first and third trimester placentae, with abundant signals for the GnRH and GnRH-R message both in the cyto- and syncytiotrophoblasts. Single trophoblasts of different gestational ages in culture also displayed GnRH expression in individual cytotrophoblasts and in syncytiotrophoblast-like fusionates. Additional immunostaining revealed GnRH peptide to be co-localized with GnRH-R message in trophoblast layers. Since messages for GnRH and GnRH-R were found in virtually all trophoblasts, we infer that GnRH and GnRH-R are co-expressed in identical cells. These data strongly suggest that the trophoblasts are the source of GnRH, and that there is autocrine/ paracrine regulation by GnRH in human placenta.