Several forms of gonadotropin-releasing hormone (GnRH)-like molecules were found in brains of both anurans (frogs) and urodeles (salamanders). The presence of the mammalian-like GnRH molecule was confirmed by HPLC and cross-reactivity studies. Small amounts of salmonid-like GnRH molecules in the brains of frogs (Rana pipiens, Hyla regilla) and salamanders (Taricha granulosa, Ambystoma gracile) were detected by comparing the HPLC chromatographic pattern and immunological reactivity of the brain extracts with native trout and synthetic salmon GnRH. This nonmammalian form of GnRH in the amphibian brain is similar and perhaps identical, at least by indirect evidence, to a form of GnRH reported earlier to be in sympathetic ganglion, retina, chromaffin tissue, and tadpole brain. If two of the amphibian GnRH molecules prove to be mammalian and salmon GnRH, then it is likely that two separate genes in amphibians code for the distinct primary structures of the molecules. The most parsimonious interpretation of the presence of both mammalian- and salmon-like GnRH in anurans and urodeles is that a common phylogenetic ancestor also possessed the two forms of GnRH. Thus the mammalian form of GnRH may well have been present in labyrinthodont amphibians. Independent of evolutionary origin, the functions of the different GnRH molecules in amphibians are unknown.