The aim of the present study was to quantify the density and affinity of human extrastriatal dopamine D2 receptors using positron emission tomography (PET). [(11)C]FLB-457, a high-affinity dopamine D2 receptor antagonist with various specific radioactivities (SA) was used. Eight healthy male subjects, age 20-35 yr, participated twice or three times at different SAs (1-279 GBq/ µmol), and serial dynamic scans were performed in the 3D data acquisition mode. The peak of the specific binding was not well defined with high SA due to the flatness of the curves after 60 min but was observed within the PET measurement. In the experiment with low SA, the peak came earlier than that with high SA. Scatchard analysis was performed using the maximal specific binding value (transient equilibrium) and the radioactivity in the cerebellum as free ligand concentration. The highest density was observed in the thalamus (2.3+/-0.6 pmol/ml), followed by the temporal cortex (1.5+/-0.5 pmol/ml), hippocampus (1.4+/-0.5 pmol/ml), parietal cortex (0.9+/-0.4 pmol/ml), frontal cortex (0.8+/-0.2 pmol/ml) and occipital cortex (0.7+/-0.3 pmol/ml). There was no significant difference in K(d) values in these six regions. The present results demonstrate that dopamine D2 receptor densities in the extrastriatal regions were only 2-8% of that in the striatum. Although the density of extrastriatal dopamine D2 receptor was low, significant regional differences were observed in the present study, as reported in postmortem studies.