 PET studies have demonstrated a therapeutic window of dopamine D2 receptor occupancy 60-80 in patients with schizophrenia. This observation has been used to predict the therapeutic dose range. Ziprasidone is recommended to be dosed twice daily, based on a knowledge of its half life in plasma level 5-10 hours. However, the plasma kinetics do not always mirror the central kinetics. Antipsychotics with a high-affinity at D2 receptors like risperidone attach for a relatively long time to their binding sites even after plasma levels declined. Based on this observation, another antipsychotic with a similar high-affinity at D2 receptors, ziprasidone, would also be expected to keep a sufficiently high D2 receptor occupancy even 24 hours after the last dose. Given 60 D2 occupancy is required to maximize chance of therapeutic efficacy, it would be valuable to assess the D2 receptor occupancy 24 hours postdose to predict the therapeutic effects of once-daily regimen. 