The characterization of neuroreceptor functions in the living human brain has hitherto been hampered by the lack of techniques useful for the measurement of physiologic events within the human brain in vivo. Recent developments in positron emission tomography (PET) has allowed the quantitative tracing of intravenously administered molecules within tissue compartments of the brain. Using ligands binding with high affinity to cerebral neuroreceptors it has been possible to examine not only the distribution but also some quantitative aspects of brain neuroreceptors in living human subjects by the PET technique. By the selection of highly selective ligands for different receptor systems and by labeling them with positron emitting isotopes as 11C or 18F, methods have been developed for the characterization of receptor subtypes for the endogenous ligands dopamine, serotonin, opiates, acetylcholine, glutamate and GABA. The present communication describes the use of 11C-SCH 23390 and 11C-raclopride for the analysis of D1 and D2 dopamine receptors, the benzodiazepine (BZ) antagonist 11C-Ro 15-1788 for benzodiazepine receptors and 11C-nicotine for nicotine receptors. Specificity of binding was verified by using active and inactive stereoenantiomers of the ligands. After the intravenous administration of the ligands quantitative aspects of the receptor binding was calculated using models according to equilibrium or dynamic approaches. Bmax and Kd values for D2 dopamine and BZ receptors could be determined in the brain of healthy human subjects and patients with neuropsychiatric disorders. No alteration of D2 dopamine receptors in the major basal ganglia were found in drug naive schizophrenic patients.(ABSTRACT TRUNCATED AT 250 WORDS)