We isolated and characterized two distinct cDNAs for mammalian gonadotropin-releasing hormone (mGnRH) and chicken GnRH-II (cGnRH-II) precursors from the Japanese eel by rapid amplification of cDNA ends. Each GnRH precursors were composed of a signal peptide, a GnRH decapeptide, a processing site and a GnRH-associated peptide. Northern blot and reverse transcription-polymerase chain reaction analysis revealed that the mGnRH precursor gene is expressed in all tissues tested including the brain, pituitary, eye, olfactory epithelium, ovary, testis, liver, kidney, spleen, heart, gill, intestine, pancreas, muscle, skin, fin and peripheral blood leukocyte. In contrast, the cGnRH-II precursor gene expression was detected only in the brain, pituitary, olfactory epithelium, ovary and testis. These findings suggest unknown physiological function(s) for mGnRH besides the well-documented role in the pituitary gonadotropin synthesis and release. The eel mGnRH and cGnRH-II precursors have high amino acid homologies with seabream GnRH (sbGnRH) precursors of the Perciforms and cGnRH-II precursors of other teleosts, respectively. Phylogenetic analysis showed the existence of three distinct evolutionary arms of GnRHs; multiple GnRH forms (mGnRH, guinea pig GnRH, chicken GnRH-I, sbGnRH and catfish GnRH (cfGnRH)) on the first, cGnRH-II on the second, and salmon GnRH (sGnRH) on the third arm. This analysis suggests that mGnRH progenitor has undergone sequence divergence to give rise to sbGnRH and cfGnRH, whereas sGnRH represents a separate evolutionary line.