Very little information has been published describing the relationship between atypical antipsychotic plasma concentrations and dosages. However, it is very likely that most or all atypical antipsychotics have a linear relationship between typical dosages and concentrations (first-order kinetics), particularly for the same patient. Pharmacologists use an easy formula, the concentration-dose ratio (C/D), to represent this relationship. According to the limited data that has been published from one of the risperidone multicenter studies (1), the average total risperidone plasma concentration, which was calculated by adding risperidone and 9-hydroxyrisperidone concentrations, for patients who were taking 2 mg a day was 14 ng per ml (C/D=7.0). For patients taking 6 mg a day, it was 43 ng per ml (C/D=7.2); for those taking 10 mg a day, it was 73 ng per ml (C/D=7.3); and for those taking 16 mg a day, it was 111 ng per ml (C/D=6.9). Therefore, the average C/D for risperidone is approximately 7.