Clozapine (Leponex®) has been shown to be therapeutically effective in patients resistant to long-term medication with classical antipsychotics. The mode of action of clozapine is not clear, but several cerebral receptors have been implicated, including the dopamine D2, D3 and D4 types, α-adrenergic, serotonin (type 2A) and glutamate (NMDA-type) receptors. Moreover, clozapine has anti-cholinergic and antihistaminergic potencies. Thusfar, receptor profiles are based virtually exclusively on in vitro binding assays. It appeared, that pharmacological and physiological stimuli activate particular gene expression, in vivo, so at cellular level the action of e.g. antipsychotics can now be traced. In this communication we present data on the in vivo profile of clozapine as revealed with Fos-protein expression. The immediate early gene c-fos is, as other members of the class of such genes, rapidly and transiently induced in the brain. The prototypic members of this class all encode nuclear proteins that regulate gene transcription. Recent studies have shown that the antipsychotics haloperidol (Haldol®) and clozapine, when given acutely, induce different patterns of Fos-like immunoreactivity in the forebrain of the rat. The most marked effects of haloperidol were found in the striatum, the nucleus accumbens and the lateral septum.