Immunohistochemical localization of cytochrome P-450 in the brains of normal untreated male rats has been demonstrated with polyclonal antibodies raised against hepatic P-450C. These antibodies recognize P-450C and D, two closely related forms of P-450. We have identified P-450C,D immunoreactivity (P-450C,D-i) within both neurons and glial cells. Neuronal cell bodies showing P-450C,D-i were present in several forebrain areas, including the olfactory bulb, the caudate nucleus, the cingulate, the frontal and the entorhinal cortex, the hippocampus and the thalamus. In the brainstem, P-450C,D-i neurons were detected in the substantia nigra, the nucleus locus coeruleus, raphe nucleus, reticular formation and the trigeminal nerve nucleus and the dorsal motor nucleus of the vagus. Glial cells containing P-450C,D-i were situated within myelinated fiber tracts in the forebrain (e.g. the corpus callosum, the internal capsule) and the brainstem (e.g. the superior cerebellar peduncle, the medial longitudinal fasciculus). Some, but not all, of the P-450C,D-i glial cells also stained with an antibody against glial fibrillary acidic protein. The present findings represent the first immunohistochemical demonstration of cytochrome P-450 in the brain and suggest that cytochrome P-450C,D is constitutive in neurons and glial cells of most brain regions throughout the rat neuraxis.