We examined the characteristics of [3H]clozapine binding sites in four rat brain regions (frontal cortex, limbic area, hippocampus and striatum) in order to elucidate the pharmacological profile of this unique atypical antipsychotic drug. The specific [3H]clozapine binding was found to be saturable and reversible in all these brain regions. Scatchard analysis of the saturation data indicated that the specific binding consisted of high- and low-affinity components. Displacement experiments showed that the muscarinic cholinergic receptor represented about 50% of [3H]clozapine binding in each brain area. Serotonin 5-HT2 and dopamine D4 receptor binding sites could also be detected by displacement experiments using ketanserin and nemonapride, respectively, in frontal cortex and limbic area, but not in hippocampus or striatum. Alpha-1, alpha-2, histamine H1, dopamine D1, D2, or D3 receptor components could not be determined within the high-affinity [3H]clozapine binding sites in any brain region. It is possible that the atypical property of clozapine may depend on the modulatory effect on dopaminergic function via 5-HT2 receptor blockade and/or may be mediated via D4 receptor blockade in the mesocortical and mesolimbic area.