In vivo occupation of dopamine D1 and D2 and serotonin (5-HT)2 receptors by typical and atypical antipsychotic drugs (APD) was examined using N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline, which nonselectively and irreversibly inactivates these receptor sites. APD were classified as typical or atypical based on their capacity to induce extrapyramidal side effect in humans and/or catalepsy in laboratory rodents. Pretreatment of rats with typical APD (haloperidol, 0.25-3 mg/kg; chlorpromazine, 5-10 mg/kg; cis-flupenthixol, 1 mg/kg; zotepine, 5 mg/kg; nemonapride, 0.5-2 mg/kg) potently reversed the N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline-induced D2 receptor inactivation in rat striatum. In contrast, some atypical APD or its candidates (clozapine, 5-30 mg/kg; fluperlapine, 10 mg/kg; risperidone, 0.25-3 mg/kg; setoperone, 0.025-0.25 mg/kg; ORG 5222, 0.25 mg/kg) showed considerable occupation of 5-HT2 receptors in cerebral cortex with smaller or negligible occupation of D2 and D1 receptors. Pretreatment with the other atypical APD (sulpiride, 30 mg/kg; amperozide, 1 mg/kg) had no effect on these three receptors, although at higher doses, sulpiride (60 mg/kg) and amperozide (5 mg/kg) slightly but significantly reversed D2 and 5-HT2 receptor inactivation, respectively. It was concluded that a certain group of atypical APD is characterized by high occupancy of 5-HT2 receptor with lower or minimal occupancy of D2 and D1 receptors in vivo. The relevance of these characteristics of atypical APD was discussed in relation to extrapyramidal side effects and the therapeutic effects on schizophrenia.