The expression of prepro-catfish GnRH mRNA and prepro-chicken GnRH-II mRNA was investigated by means of in situ hybridization. The differential distribution of cells expressing the respective mRNAs was compared with the distribution of cells immunoreactive for (1) catfish (cf) GnRH and chicken (c) GnRH-II and (2) both GnRH-associated peptides (GAPs). It was found that the prepro-cfGnRH mRNA expressing cells were located in the ventral forebrain, with a similar distribution of the cfGnRH- and cfGAP-immunoreactive perikarya. The prepro-cGnRH-II mRNA expressing cells were exclusively located in the midbrain tegmentum, at the same position as a group of large cGnRH-II- and CIIGAP-immunoreactive perikarya. It was concluded that the peptidergic neurons in the ventral forebrain contain cfGnRH, whereas cGnRH-II perikarya are restricted to the midbrain. The proximal pars distalis of the pituitary, containing the gonadotropin cells, is innervated by fibers immunoreactive for both cfGnRH and cfGAP and originating from the cfGnRH neurons in the ventral forebrain. We could, however, not detect fibers innervating the pituitary that were immunoreactive for cIIGAP.