Several products of the hepatic metabolism of clozapine are found in high concentrations in the plasma of schizophrenic patients treated with this atypical antipsychotic drug. One of these metabolites, N-desmethylclozapine, has substantially different affinities for dopamine and serotonin metabolites than does the parent compound. However, it is not known if this metabolite is active in vivo. We examined the effect of acute administration of desmethylclozapine to rats on forebrain Fos protein expression. Clozapine induces expression of this immediate-early gene in a distinct regional pattern in the brain. Desmethylclozapine significantly increased Fos protein expression in the medial prefrontal cortex and nucleus accumbens, but not in the dorsolateral striatum, thus mirroring the effects of the parent compound. These data indicate that the desmethyl metabolite of clozapine has in vivo biological activity.