Cytochrome P450s are enzymes involved in the oxidative metabolism of numerous endogenous and exogenous molecules. The enzyme cytochrome debrisoquine/sparteine-type monoxygenase is a specific form of cytochrome P450 and is found in the liver and the brain (in the rat the enzyme is known as CYP2D1). CYP2D1 has no established role in the brain; however, it has been shown to share substrate and inhibitor specificities with the dopamine transporter and the enzyme monoamine oxygenase B. Using CYP2D-specific deoxyoligonucleotide probes and a polyclonal antibody to CYP2D1, we have mapped the distribution of CYP2D mRNA and CYP2D1-like immunoreactivity in the rat central nervous system. CYP2D1 immunoreactivity and the CYP2D1 mRNA signal were heterogenously distributed between brain areas. There were moderate to high levels of immunoreactivity and mRNA signal in the olfactory bulb, olfactory tubercle, cerebral cortex, hippocampus, dentate gyrus, piriform cortex, caudate putamen, supraoptic nucleus, medial habenula, hypothalamus, thalamus, medial mammilliary nucleus and superior colliculus. In the brainstem, strong CYP2D1 immunoreactivity and CYP2D mRNA signal were observed in the substantia nigra compacta, red nucleus, interpeduncular nucleus, pontine grey, locus coeruleus, cerebellum, and the ventral horn of the spinal cord. This study indicates that CYP2D1 is widely and constitutively expressed in neuronal and some glial populations in the rat brain. The localization of CYP2D1 in several regions known to harbor catecholamines and serotonin may suggest a role for CYP2D1 in the metabolism of monoamines.