The assessment of immediate-early gene induction has proven to be a useful method for delineating the neural systems that subserve antipsychotic drug actions. In order to differentiate the sites and mechanisms of action of typical and atypical antipsychotic drugs, we examined the effects of antipsychotic drugs on Fos protein expression in the medial prefrontal cortex. The atypical antipsychotic drug clozapine selectively increased the number of neurons that expressed Fos-like immunoreactivity in the prefrontal cortex, targeting the deep layers of the infralimbic and prelimbic cortices. Pyramidal cells were the major cell type in which Fos was expressed. A small number of calbindin-like immunoreactive, but not parvalbumin- or reduced nicotinamide adenine dinucleotide phosphate diaphorase-containing, interneurons also expressed Fos after clozapine challenge. Immunoblot studies revealed that clozapine induced Fos protein in the infralimbic and prelimbic cortices. Other antipsychotic drugs that are D2 receptor antagonists, including haloperidol, raclopride, sulpiride, remoxipride and loxapine, did not alter Fos expression. The clozapine-induced increase in Fos expression was also not attributable to actions at the D1 dopamine receptor, nor to serotonin type 2a/2c receptor antagonism or combined serotonin type 2-D2 dopamine receptor antagonism. The ability of clozapine to block alpha 1-adrenergic or muscarinic cholinergic receptors did not contribute to the unique actions of clozapine. Despite the inability of dopamine receptor antagonists other than clozapine to elicit an increase in Fos expression, both the mixed D1-D2 dopamine agonist apomorphine and the D2-like agonist quinpirole increased Fos protein levels in the prefrontal cortex. However, neither pretreatment with sulpiride to block D2/3/4 dopamine receptors or SCH 23390 to block D1/5 dopamine receptors modified the Fos response to clozapine. Since dopamine receptor antagonist pretreatments did not attenuate the clozapine-elicited Fos expression, but D2 agonists increased cortical Fos expression, clozapine may act in the prefrontal cortex on an as yet undefined dopamine receptor. In contrast to the nucleus accumbens shell, where all antipsychotic drugs increase Fos expression, only clozapine induced Fos in the medial prefrontal cortex. These observations suggest that the ability of clozapine to treat schizophrenic patients who are resistant to the therapeutic benefits of conventional antipsychotic drugs may occur through actions in the prefrontal cortex.