The first set of the present experiments was designed to investigate the postnatal development of corticotropin releasing factor-like immunoreactivity (CRFI) in the rat cerebral cortex by means of cobalt-enhanced immunohistochemistry. Results showed the occurrence of CRFI fibres before cells in the developing rat cerebral cortex with and without colchicine treatment, suggesting that some CRFI cells in subcortical regions may project to the cerebral cortex. In the second set of experiments, ipsilateral double-labelled cells which contained both retrogradely transported horseradish peroxidase (HRP) and CRFI were observed in the zona incerta, subincertal nucleus, lateral hypothalamic area, perifornical hypothalamic area, and in the dorsal hypothalamic area after unilateral HRP injections into the cerebral cortex. These findings indicate the existence of corticopetal CRFI-containing projections arising from the above areas.