Dysregulation of the 5-HT_2A receptor is implicated in both the etiology and treatment of schizophrenia. Although the essential role of 5-HT_2A receptors in atypical antipsychotic drug actions is widely accepted, the contribution of 5-HT_2A down-regulation to their efficacy is not known. We hypothesized that down-regulation of cortical 5-HT_2A receptors contributes to the therapeutic action of atypical antipsychotic drugs. To test this hypothesis, we assessed the effect of chronically administered antipsychotics (clozapine, olanzapine, and haloperidol) and several 5-HT_2A antagonists [ketanserin, altanserin, α-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidinemethanol (M100907), α-phenyl-1-(2-phenylethyl)-4-piperidinemethano (M11939), 4-[(2Z)-3-{[2-(dimethylamino)ethoxy]amino}-3-(2-fluorophenyl)prop-2-en-1-ylidene]cyclohexa-2,5-dien-1-one (SR46349B), and pimavanserin], on the phencyclidine (PCP)-induced hyperlocomotor response and cortical 5-HT_2A receptor levels in C57BL/6J mice. Clozapine and olanzapine, but not haloperidol, induced receptor down-regulation and attenuated PCP-induced locomotor responses. Of the selective 5-HT_2A antagonists tested, only ketanserin caused significant receptor protein down-regulation, whereas SR46349B up-regulated 5-HT_2A receptors and potentiated PCP-hyperlocomotion; the other 5-HT_2A receptor antagonists were without effect. The significance of these findings with respect to atypical antipsychotic drug action is discussed.