Gonadotroph tumors predominantly secrete FSH or free gonadotropin hormone subunits and rarely LH. In contrast to normal gonadotrophs, a subset of tumors synthesize FSH beta-subunit (SU) in excess of alpha-SU, and the cause of gonadotropin hormone-SU biosynthetic defects in these tumors is unknown. Gonadotropin-releasing hormone (GnRH) is known to modify gonadotropin hormone-SU biosynthesis and secretion and may be an important determinant of gonadotroph tumor hormone regulation. Data in experimental animals have demonstrated that endogenous expression of GnRH may occur in the pituitary. We therefore determined whether 1) the GnRH gene is expressed in gonadotroph tumors and normal pituitaries using reverse transcriptase (RT)-PCR; 2) the GnRH receptor gene is co-expressed in gonadotroph tumors; 3) an alternative upstream transcriptional start site on the GnRH gene is utilized; and 4) media from primary cultures of gonadotroph tumors have detectable GnRH immunoreactivity by RIA. GnRH messenger RNA (mRNA) was detected in 10/10 gonadotroph tumors, eight of which expressed GnRH-Receptor mRNA. Both mRNAs were detected in all normal pituitaries studied (n = 6). Six of 10 gonadotroph tumors and 3/6 normal pituitaries had GnRH transcripts derived from the upstream transcriptional start site (5'GnRH). GnRH immunoreactivity was detected in overnight media from 3/3 primary gonadotroph tumor cultures (range: 1.89-5.86pg/mL; limit of detection (LD) = < 1.58pg/mL) but was not detectable in control media. This is the first study to demonstrate endogenous GnRH gene expression in human pituitary adenomas and normal human pituitary tissue. The presence of both GnRH and GnRH-Rc suggest that GnRH may be a paracrine/autocrine regulator of cell function in the pituitary and may affect gonadotroph tumor hormone phenotype.