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Objective: Long-acting injectable risperidone represents the first clinically available depot atypical antipsychotic. The present study used positron emission tomography (PET) to evaluate its dopamine D 2 binding profile at doses of 25, 50, or 75 mg administered every 2 weeks. Method:After achieving stabilization with one of the doses, nine patients with a diagnosis of schizophrenia or schizoaffective disorder underwent [ 11 C]raclopride PET to measure D 2 occupancy. Participants were scanned twice during the 2-week injection interval: within 3 days after injection (postinjection) and within 5 days before the next injection (preinjection). At the same time, plasma was collected for measurements of risperidone plus 9-hydroxyrisperidone.Results: Mean post-and preinjection D 2 occupancy levels for the 25-, 50-, and 75-mg doses were 71.0% and 54.0%, 74.4% and 65.4%, and 81.5% and 75.0%, respectively. There was a significant correlation between dose and plasma concentrations of risperidone plus 9-hydroxyrisperidone, and the estimated plasma concentration asso ciated with 50% D 2 occupancy (ED 50 ) was 11.06 ng/ml. Prolactin levels were not correlated with drug levels or D 2 occupancy.Conclusions: All three doses of injectable risperidone showed peak D 2 occupancy levels above the 65% threshold associated with optimal clinical response; the 75-mg dose approximated the 80% threshold linked to increased risk of extrapyramidal symptoms. Doses of 25 or 50 mg should provide therapeutic efficacy while minimizing the risk of extrapyramidal symptoms.