In situ radioligand binding and quantitative autoradiography have been used to measure the density of striatal D1-like, D2-like, and GABAA receptors in rats treated with haloperidol at 0.01 or 0.1 mg/kg/ day or chlorpromazine, olanzapine or clozapine at 0.1 or 1.0 mg/kg/day for 1, 3 or 7 months. [3H]SCH23390 binding to D1-like receptors was not changed by any drug treatments. There were significant increases in [3H]nemonapride binding to D2-like receptors at different time points due to treatment with haloperidol, chlorpromazine and olanzapine. By contrast, treatment with clozapine and olanzapine caused a time-dependent decrease in [3H]muscimol binding to the GABAA receptor. These data suggest that treatment with atypical antipsychotic drugs, but not typical antipsychotic drugs, affect striatal GABAergic neurons. In addition, it would appear that clozapine might be unique in that it does not increase dopamine-D2 like receptor density at doses which would be predicted to have antipsychotic effects in humans. The extent to which such changes are involved in the therapeutic effects of drugs such as olanzapine and clozapine remains to be determined.