The cannabinoid receptor family consists of two inhibitory G-protein-coupled receptors, CB1 and CB2. CB1 is distributed primarily in neural tissue, whereas CB2 is distributed predominantly in immune cells. The distribution of cannabinoid receptors in neural tissue has been demonstrated by using ligand binding autoradiography with CP55,940, a high-affinity cannabinoid receptor ligand, and in situ hybridization. However, the localization of CB1 within individual cells in the brain remains to be defined. In the present study, domain-specific polyclonal antibody to amino acids 83-98 of CB1 was used to define the expression of the neural cannabinoid receptor at the histochemical level. The use of CB1-specific antiserum is advantageous in view of recent reports that CB2 also is expressed in the brain and binds CP55,940. Thus, utilization of anti-CB1 antiserum would allow for the specific detection of CB1 protein expression. The regional staining pattern for CB1 in rat brain was consistent with that reported for CB1 using ligand binding autoradiography and in situ hybridization. Intense immunoreactivity was present in the hippocampal formation, the basal ganglia, and the molecular layer of the cerebellum. Moderate immunohistochemical staining was observed in the olfactory bulb, piriform cortex, cerebral cortex, and the granular layer of the cerebellum. In addition, immunoreactive staining was concentrated on afferent projections and dendritic processes of neuronal cells and was present within cell bodies and on cell surfaces. These data indicate that the anti-CB1 antibody is a sensitive probe for the unequivocal histological discrimination of CB1 protein expression.