Central D2-dopamine receptor occupancy was followed by repeated PET experiments after administration of single oral doses of haloperidol to four healthy men. D2-dopamine receptor occupancy was high already 3 h after administration of 4 and 7.5 mg haloperidol and remained high for at least 27 h. Akathisia appeared when D2-dopamine receptor occupancy was maximal. After initiation of neuroleptic drug treatment several days or weeks may elapse before antipsychotic effect is evident. The results of this study do not indicate that any late onset of the antipsychotic effect is related to an insufficient D2-dopamine receptor occupancy during the first days of treatment.