GnRH has been shown to play a role in the regulation of secretion of hCG by human placenta. Immunocytochemical studies have demonstrated the presence of the peptide, but do not address whether the GnRH is maternal, fetal, or placental in origin. In situ hybridization studies using a biotinylated pro-GnRH cDNA were, therefore, undertaken to determine the distribution of pro-GnRH mRNA in first trimester placental samples. Using an avidin-biotin-Cy.5 detection system in conjunction with laser scanning confocal microscopy, pro-GnRH mRNA was shown to be present in both the cytotrophoblast and syncytiotrophoblast cells of placental villi, although not in the cells of the fetal connective tissue. Prior hybridization with a 200-fold excess of unlabeled probe blocked hybridization of the labeled pro-GnRH probe. Southern and sequence analysis demonstrated that the probe hybridized to a transcript identical to hypothalamic GnRH. Immunocytochemical staining using an antiserum to amino acids 6-16 of pro-GnRH demonstrated the presence of translated pro-GnRH in both the cytotrophoblast and syncytiotrophoblast epithelia. We conclude that the synthesis of pro-GnRH by both the cytotrophoblast and the syncytiotrophoblast of human placenta is consistent with either an autocrine or a paracrine mode of GnRH regulation of hCG secretion.