Immunohistochemical distribtttion of cannabinoid receptors in the adult rat brain was studied using specific purified antibodies against the amino-terminus of the CB1 receptor. our results generally agree well with the previous studies using CB1 receptor au-toradiography and mRNA in situ hybridization. However, because of its greater resolution, immunohistochemistry allowed identification of particuar neuronal cells and fibers that pos-sess cannabinoid receptors. CB 1-Iike immunoreactivity was found in axons, cell bodies and dendrites, where it appeared as puncta in somata and processes. Both intensely and moder-ately or lightly stained netlrons were observed. The intensely stained neurons were dispersed and only occur in cortical structures incltlding hippocampal formation and olfactory bulb.Moderately or lightly stained netIrons were found in caudate-putamen and amygdala. In the hippocampal formation only intensely stained neurons were observed. The cell bodies of pyramidal neurons in CAl and CA3 fields appeared to be unstained but surrounded by a dense plexus of immunoreactive fibers. The granLIle cells in the dentate area were also im-munone-gative. Many intensely stained neurons were located at the base of the granule cell layer. CB 1-like immunoreactive neurons and fibers were also found in the somatosensory,cingulate, perirhinal, entorhinal and piriform cortices, in claustrum, amygdaloid nuclei, nu-cleus accumbens and septtlm. Beaded immunoreactive fibers were detected in periaqueductal gray, nucleus tractus solitarius, spinal trigeminal tract and nucletls, dorsal horn and lamina X of the spinal cord. A triallgular cap-like mass of immunoreactivity was found to surround the basal part of the Purkinje cell body in the cerebellum. only small, lightly stained cells were found in the molecular layer in the cerebellum close to the Purkinje cell layer.The CB 1 receptor is widely distribtlted in the forebrain and has a more restricted distribu-tion in the hindbrain and the spinal cord. It appears to be expressed on cell bodies, dendrites and axons. According to the location and morphology, many, but not all, CB1-like im-munoreactive neurons appear to be GABAergic. Therefore, cannabinoicls and cannabinoid re-ceptors may play a roIe in modulating GABAergic neurons. Immunohistochemical distribtttion of cannabinoid receptors in the adult rat brain was studied using the specific purified antibodies against the amino-terminus of the CB1 receptor. Our results generally agree well with the previous studies using CB1 receptor au-toradiography and mRNA in situ hybridization. However, because of its greater resolution, immunohistochemistry allowed identification of particuar neuronal cells and fibers that pos-sess cannabinoid receptors. CB 1 -Iike immunoreactivity was found in axons, cell bodies and dendrites, where it appeared as puncta in somata and processes. Both intensely and moder -ately or lightly stained netlrons were observed. The intensely stained neurons were dispersed and only occur in cortical structures incltlding hippocampal formation and olfactory bulb. Moderately or lightly stained netIrons were found in caudate-putamen and amygdala. In the hippocampal formation only intensely stained neurons were observed. The cell bodies of pyramidal neurons i The CAI and CA3 fields appeared to be unstained but surrounded by a dense plexus of immunoreactive fibers. The granlIle cells in the dentate area were also im-munone-gative. Many intensely stained neurons were located at the base of the granule cell layer. CB 1-like immunoreactive neurons and fibers were also found in the somatosensory, cingulate, perirhinal, entorhinal and piriform cortices, in claustrum, amygdaloid nuclei, nu-cleus accumbens and septtlm. Beaded immunoreactive fibers were detected in periaqueductal gray, nucleus tractus solitarius, spinal trigeminal tract and nucletls, dorsal horn and lamina X of the spinal cord. A triallgular cap-like mass of immunoreactivity was found to surround the basal part of the Purkinje cell body in the cerebellum. only small, lightly stained cells were found in the molecular layer in the cerebellum close to the Purkinje cell layer. The CB 1 receptor is widely distribtlted in the forebrain and has a more restricted distribu tion in the hindbrainIt appears to be expressed on cell bodies, dendrites and axons. According to the location and morphology, many, but not all, CB1-like im-munoreactive neurons appear to be GABAergic. Thus, cannabinoicls and cannabinoid re- ceptors may play a roIe in modulating GABAergic neurons.