Corticotropin-releasing factor (CRF) has been demonstrated in the hypothalamic-pituitary-adrenal axis and studied in extrahypothalamic sites throughout the brain. Localization of CRF immunoreactivity, CRF mRNA, and CRF receptors within specific brain areas supports an extrahypothalamic function for CRF. Previous reports have revealed the localization of several peptides, including CRF, in the cerebellar cortex and nuclei of the North American opossum (Didelphis marsupialis virginiana); climbing fibers, mossy fibers, and a beaded plexus of axons in the Purkinje cell layer demonstrate CRF immunoreactivity. CRF also is localized within neurons in the inferior olivary complex and other brainstem nuclei which are known to project to the cerebellar cortex. Physiological recordings indicate CRF potentiates the excitatory effects of both aspartate and glutamate, the putative transmitters of the major afferent inputs to the cerebellum. The present study reports that specific CRF binding sites are present in all lobules of the opossum cerebellar cortex, with the greatest density in vermal lobules V through X, the flocculus, and the paraflocculus. The cerebellar nuclei do not appear to be labeled. CRF binding sites are present over all neuronal layers of the cerebellar cortex. The presence of CRF immunoreactivity in climbing fibers, mossy fibers, and a beaded axonal plexus, and CRF binding sites within the cerebellar cortex, as well as the fact that CRF potentiates the excitatory effects of both aspartate and glutamate, indicate that this peptide may function as a neuromodulator in the cerebellum of the North American opossum. An attempt is made to correlate the distribution of CRF-IR neurons and fibers, CRF mRNA, and CRF receptors to the extrahypothalamic function of CRF as it relates to the olivocerebellar pathway.