The origins of the different populations of gonadotropin-releasing hormone (GnRH)-containing neurons in the brains of two genotypes (HO4C; HNI-II) of medaka Oryzias latipes were analyzed at different stages of development (day 1 after fertilization through adulthood), by using oligonucleotide probes specific to salmon-, seabream-, and chicken II-GnRH mRNA and antisera against specific GnRH peptides. Between the two genotypes, there was no difference in the site and time of GnRH expression or the final pattern of GnRH neuronal organization. In the adult fish of both sexes, salmon GnRH mRNA and peptide-containing neurons were seen in the terminal nerve ganglia (nucleus olfactoretinalis; NOR) and chicken II-GnRH mRNA and peptide-containing neurons in the midbrain tegmentum. GnRH cells at the base of the olfactory placode (1-2 cells) and in the midbrain tegmentum were first seen in 1-day-old fish of both genotypes. On day 15, lightly immunoreactive GnRH cells were seen in the NOR of only HNI genotype. By day 30, GnRH expression in the NOR and in the midbrain was prominent. GnRH cells along the basal olfactory bulb and basal telencephalon were occasionally seen in animals 30 days or older. This developmental study shows differential distribution of salmon and chicken II-GnRH mRNA subtypes and emphasizes their separate embryonic origins from the olfactory apparatus (salmon-GnRH) and the ependymal cells of the third ventricle (chicken II-GnRH). The absence of preoptic GnRH hybridization signals, immunoreactivity and the lack of GnRH fibers in the pituitary suggests that the preoptic GnRH neurons are distinct from the olfactory derived-terminal nerve GnRH neurons, and that the GnRH neurites reported in the pituitary of teleost must be of preoptic origin.