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The effects of 3-week treatment with a typical antipsychotic drug chlorpromazine and three atypical antipsychotic drugs (risperidone, olanzapine and perospirone) on the binding to dopamine D2 and serotonin 5-HT2A receptors were examined in the rat striatum and frontal cortex, respectively. Subchronic treatment with chlorpromazine (10 mg/kg) and perospirone (1 mg/kg) significantly increased D2 receptors, while no increase was observed with lower dose of chlorpromazine (5 mg/kg), perospirone (0.1 mg/kg), risperidone (0.25, 0.5 mg/kg) or olanzapine (1, 2 mg/kg). On the other hand, 3-week administration of chlorpromazine (5, 10 mg/kg) and olanzapine (1, 2 mg/kg) significantly decreased 5-HT2A receptors, but risperidone (0.25, 0.5 mg/kg) or perospirone (0.1, 1 mg/kg) had no effect. The measurement of in vivo drug occupation for D2 and 5-HT2A receptors using N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) suggested that high occupation of 5-HT2A receptors with lower D2 receptor occupancy might be involved in the absence of up-regulation of D2 receptors after subchronic treatment with some atypical antipsychotic drugs.