Three molecular species of gonadotropin releasing hormone (GnRH) mRNA-containing neuronal populations (terminal nerve: salmon-GnRH; preoptic area: seabream-GnRH; midbrain: chicken-GnRH-II) have been localized in teleosts. While the terminal nerve GnRH neurons originate from the olfactory placode a separate source of origin for preoptic and midbrain neurons is possible. The preoptic GnRH neurons are regulated by gonadal steroids and gonadal maturation, however, the role and regulation of terminal nerve and midbrain GnRH neurons in teleosts reproduction is speculative and debatable.