 Recently, the adverse effect of antipsychotic medications on systemic glucose regulation has received increased attention as investigators noted prominent adverse glucoregulatory effects associated with certain newer antipsychotic medications. Abnormal glucose regulation and new-onset type 2 diabetes have been reported during clozapine and olanzapine treatment. Complicating the study of antipsychotic-induced changes in glucose regulation, increased adiposity can decrease insulin sensitivity, and antipsychotics can increase adiposity and body mass index BMI. However, abnormal glucose regulation and type 2 diabetes can occur during clozapine treatment in the absence of weight gain, suggesting that changes in glucose regulation can occur independent of drug-induced increases in BMI. Consistent with this, our preliminary studies indicate that important effects of clozapine and olanzapine on glucose regulation are not accounted for by differences in BMI. This proposal will compare the effects of olanzapine, risperidone and haloperidol on well-defined measures of glucose regulation. 