The neuronal distribution of cannabinoid receptor in the adult rat brain is reported, combining receptor binding radioautography using the synthetic psychoactive cannabinoid ligand CP55,940 with in situ hybridization histochemistry using oligonucleotide probes complementary to rat cannabinoid receptor cDNA. In the cerebral cortex, especially in the frontal and cingulate cortex, dense binding was found in layers I and VI together with slight mRNA levels in a majority of both pyramidal and non-pyramidal-shaped neurons and of high mRNA levels in a moderate number of non-pyramidal-shaped neurons especially in layers II-III and V-VI. In the hippocampal dentate gyrus, very dense staining was found in the molecular layer together with high mRNA levels in a moderate number of hilar neurons close to the granular layer. In Ammon's horn, especially in the CA3 sector, very dense binding was found in the dendritic layers together with slight mRNA levels in the majority of the pyramidal cells and high mRNA levels in a moderate number of interneurons. In the basal ganglia, binding was very dense in the lateral putamen, substantia nigra pars reticulata, globus pallidus and entopeduncular nucleus, moderate in the medial putamen and caudate; and slight in the accumbens, together with slight to moderate mRNA levels in the striatal medium-sized neurons. Together with slight binding, slight to moderate mRNA levels were found in the majority of the neurons in the subthalamic nucleus. No binding and mRNA were found in the substantia nigra pars compacta and ventral tegmental area. Slight to moderate binding was found together with slight to moderate mRNA levels in the majority of neurons in the anterior olfactory nucleus; septum, especially medial septum and diagonal band of Broca; amygdala, especially basolateral amygdala; lateral habenula; ventromedial hypothalamic nucleus; lateral interpeduncular nucleus; central gray, dorsal cochlear nucleus; parabrachial nucleus; dorsal pontine tegmentum; pontine nuclei; commissural part of the nucleus tractus solitarius; inferior olive and dorsal horn of the spinal cord. In the cerebellum, very dense binding was found in the molecular layer together with slight mRNA levels in the majority of the granule cells and moderate mRNA levels in the basket and stellate cells. In conclusion, this study provides, for the first time, indirect assessment of the neurons containing cannabinoid receptor in the entire adult rat brain and will serve as a basis for future direct morphological confirmation using receptor immunohistochemistry and for functional studies.