Among the brain imaging techniques developed during the past two decades, positron emission tomography (PET) has the highest sensitivity allowing the analysis of specific neurotransmitter mechanisms in the living human brain. By using a combination of selective ligands labeled with positron emitting isotopes D1- and D2 dopamine, serotonin 5HT2, and benzodiazepine receptors were examined in schizophrenic patients (DSM-III-R) and healthy control subjects. With this technique receptor populations could be excellently visualized and quantified with regard to number and binding characteristics in several brain regions. The characteristics of the total D1 and D2 dopamine receptor populations in the caudate and putamen did not differ in young drug-naive schizophrenic patients and age-matched control subjects. Also for 5HT2 and benzodiazepine receptors no major alteration of receptor characteristics was observed in several neocortical and limbic brain regions. However, in schizophrenic patients treated with chemically different types of antipsychotic drugs major reductions of ligand binding was observed indicating specific induction of neuroreceptor occupancy. Thus, all chemically different types of antipsychotic drugs examined induced a substantial occupancy of D2 dopamine receptors. Clozapine in high doses induced a significantly lower degree of D2 dopamine receptor occupancy than the conventional drugs. Some but not all antipsychotics also induced a significant D1 dopamine receptor occupancy. In spite of the fact that the selective D1 antagonist SCH 39166 induced a substantial D1 occupancy, this drug did not exhibit an antipsychotic effect in schizophrenic patients. A very high degree of 5HT2 occupancy in neocortical regions was observed after clinical treatment with antipsychotic drugs as clozapine, risperidone and thioridazine.(ABSTRACT TRUNCATED AT 250 WORDS)