The interaction of risperidone, 9-hydroxyrisperidone (the principal active metabolite), and clozapine with neurotransmitter receptors was investigated in vitro using animal brain tissue homogenates and cloned human receptors expressed in cells and ex vivo using quantitative receptor autoradiography in rat and guinea pig brain sections. In vitro, risperidone and 9-hydroxyrisperidone had similar binding profiles, and their highest affinity was for 5-HT2A receptors (cloned human, Ki 0.4 nM); affinities for other 5-HT-receptor subtypes were at least 100 times lower. Risperidone bound to 5-HT2A receptors with 20 times greater affinity than clozapine and 170 times greater affinity than haloperidol. Clozapine primarily bound to histamine H1 receptors and haloperidol to dopamine D2 receptors. The binding affinity of risperidone and 9-hydroxyrisperidone for the D2 family of receptors (D2L, D2S, D3, D4) was one order of magnitude lower than their affinity for 5-HT2A receptors. Risperidone bound to D2 and D3 receptors with 50 and 20 times greater affinity than clozapine and was only 2 to 3 times less potent than haloperidol. All compounds bound with similar affinities to D4 receptors (Ki 5-9 nM), and their affinities for D1 receptors were 100 times lower than for D4 receptors. The ex vivo receptor occupancy profile of the compounds matched the in vitro receptor binding profile. A conspicuous property of risperidone, not seen for the other compounds, was the shallow occupancy curve at D2 receptors in the striatum and mesolimbic brain area. Moreover, it was observed that antagonism of strong D2-receptor stimulation by apomorphine in rats was achieved at less than 50% D2 occupancy by the antipsychotics.(ABSTRACT TRUNCATED AT 250 WORDS)