The distribution in the rat brain of fatty acid amide hydrolase (FAAH) an enzyme that catalyzes the hydrolysis of the endogenous cannabinoid anandamide was studied by immunohistochemistry. An immunopurified, polyclonal antibody to the C terminal region of FAAH was used in these studies. The large principal neurons, such as pyramidal cells in the cerebral cortex, the pyramidal cells the hippocampus, Purkinje cells in the cerebellar cortex and the mitral cells in the olfactory bulb, showed the strongest FAAH immunoreactivity. These FAAH-containing principal neurons except the mitral cells in the olfactory bulb are in close proximity with cannabinoid CB1 receptors as revealed by our previous immunohistochemical study. Moderately or lightly stained FAAH-containing neurons were also found in the amygdala, the basal ganglia, the deep cerebellar nuclei, the ventral posterior nuclei of the thalamus, the optic layer and the intermediate white layer of the superior colliculus and the red nucleus in the midbrain, and motor neurons of the spinal cord. These data demonstrate that FAAH is heterogeneously distributed and this distribution exhibits considerable, although not complete, overlap with the distribution of cannabinoid CB1 receptors in rat brain.