Previously, we demonstrated that serotonin-lesioned rats had an enhanced mesoprefrontal dopaminergic response to restraint stress. This study attempted to extend our knowledge regarding this serotonin/dopamine interaction by seeing if suppression of serotonin metabolism by chronic administration of the atypical antipsychotic, clozapine, would have similar effects. Both typical and atypical neuroleptics require chronic administration in humans before antipsychotic activity is seen. Rats treated for 21 days with clozapine or haloperidol, a typical antipsychotic without significant binding affinity for serotonergic receptors, showed lowered basal dopamine metabolism in the medial prefrontal cortex, the nucleus accumbens, and the striatum, as expected. Basal serotonin metabolism in the prefrontal cortex was also lowered by clozapine treatment, but not haloperidol. One of two challenges were given to chronically treated rats: 30 min of restraint stress or an acute challenge of clozapine. When corrected for baseline differences, both challenges significantly elevated dopamine metabolism in the prefrontal cortex of the clozapine group more than the saline or haloperidol groups. No hyperresponsiveness was seen with serotonin metabolism in the prefrontal cortex or either dopamine or serotonin metabolism in the nucleus accumbens in clozapine-treated, challenged rats. Additionally, this augmentation of the dopaminergic stress response was not seen with a single, acute administration of clozapine. The significance of the clozapine-induced hyperresponsiveness of the mesoprefrontal dopamine system is discussed with regard to clinical efficacy of clozapine.