Immunohistochemical, in situ hybridization histochemical, and Northern blot methods were used to demonstrate and characterize the distribution of corticotropin-releasing factor immunoreactivity (CRF-IR) and mRNA in the rat olfactory system. Northern analysis demonstrated the presence of an mRNA species in the olfactory bulb indistinguishable from, and in greater abundance than, CRF mRNA isolated from whole hypothalamus. Results from hybridization histochemical and immunohistochemical studies converged to indicate that CRF is expressed in a majority of mitral and tufted cells in the main and accessory bulbs, and in subsets of granule and periglomerular cells. Consistent with cellular localizations in primary output neurons, a dense network of fine CRF-immunoreactive varicosities was demonstrated in the external plexiform layer of the olfactory bulb and in layer Ia of piriform cortex. Other acknowledged terminal fields of the projection neurons of the main and accessory bulbs also displayed CRF-IR. The results suggest that CRF is the most broadly distributed neuroactive agent yet charted in olfactory bulb somata. This peptide may serve as a modulator or co-transmitter of importance in several cell types in the main and accessory olfactory bulbs, including the principal output neurons.