A monoclonal antibody generated against purified acetylcholine receptor from Torpedo electric organ was used to immunohistochemically localize a neuronal nicotinic acetylcholine receptor. Regions of the rat brain stained with this antibody paralleled those areas of the brain exhibiting [3H]nicotine binding sites and corresponded to areas in which mRNAs encoding for alpha subunits of the neuronal nicotinic acetylcholine receptor are present. Thus, the anteroventral thalamus, cortex, hippocampus, medial habenula, interpeduncular nucleus, and substantia nigra/ventral tegmental area exhibited significant immunoreactivity. Neurons of the medial habenula and substantia nigra were densely stained, and processes were prominently delineated. Furthermore, in the projection areas of the medial habenula (interpeduncular nucleus and median raphe) axons were strongly immunoreactive and were distributed to distinct subdivisions of the target sites. The present data suggest that there are several discrete neuronal systems in which nicotinic acetylcholine receptors have functional importance. These immunohistochemical studies delineate at the single-cell level the localization within the mammalian central nervous system of certain nicotinic acetylcholine receptors.