combined immunohistochemistry for gonadotropinreleasing hormone gnrh and prognrh and in situ hybridization for gnrh messenger ribonucleic acid in rat brain experiments were performed to explore the distribution of neurons containing prognrh and gnrh in the rat brain and to determine the correspondence of immunoreactive peptides and prognrh mrna using avidinbiotin immunohistochemistry on free floating vibratome sections it was found that prognrh and gnrhcontaining cells exhibited a similar distribution within the preoptic areabasal hypothalamus region within individual neurons prognrh was primarily detected in the cell soma and proximal fibers whereas the decapeptide was present in cells fibers and nerve terminals combined avidinbiotin immunohistochemistry for gnrh or prognrh peptides and in situ hybridization for prognrh mrna using a crna probe revealed that the peptides and mrna could be detected in the same cells in both male and female rats prognrh mrna was localized primarily in gnrhcontaining cells however not all immunoreactive gnrh neurons contained detectable levels of prognrh mrna and not all neurons containing prognrh mrna contained gnrh peptides in proestrous females a close correlation existed between the total number of neurons containing gnrh and those containing prognrh mrna r 08409 while in intact male rats the correlation was not as high r 056 these results document the distribution of prognrh and gnrh in the rat preoptic areabasal hypothalamus and describe the extent of colocalization of gnrh peptide and prognrh mrna in proestrous females and intact male rats further work will determine how each of the molecular components is regulated during different reproductive states