The peroxidase-antiperoxidase immunocytochemical procedure was used to study the distribution of ovine corticotropin-releasing factor (CRF) and arginine vasotocin (AVT) immunoreactivities sequentially in the same sections or in adjacent sections of the brain and pituitary of Catostomus commersoni. It was found that all CRF-immunoreactive (IR) neurons in the nucleus preopticus (NPO) also contained AVT immunoreactivity. Co-localization of both immunoreactivities was also observed in fibres forming the preoptic-pituitary tract and in the neurohypophyseal digitations, the IR-CRF and IR-AVT fibres projecting mainly to the neurointermediate lobe (NIL) of the pituitary. An additional population of exclusively IR-AVT neurons and fibres in the NPO, preoptic-pituitary tract and NIL was also observed. Exclusive CRF-immunostaining was found in neurons of the nucleus lateralis tuberis (NLT), in fibres distributed in some diencephalic nuclei and in the neurohypophyseal digitations in the region of the rostral pars distalis (RPD). These results suggest that CRF- and AVT-like substances, present in NIL fibres (probably originating in the NPO), may have an integrated role in the release of the cell products from the pars intermedia, and that the control of corticotrops in the rostral pars distalis, innervated exclusively by IR-CRF fibres (probably originating in the NLT), does not require a simultaneous presence of CRF- and AVT-like substances.