A computational method is presented to estimate in vivo receptor occupancies of frequently used antipsychotic medications, which may provide a way to determine their central nervous system (CNS) effects in humans. The method can be used to estimate occupancies of several receptor types based on the daily dose of antipsychotic medication. Estimates of D2 receptor occupancy by haloperidol based on this equation were compared with those yielded by positron emission tomography (PET) measures in humans for the same compounds and dosages: the results of this comparison validated the approach.