 This study hypothesizes that switching to aripiprazole treatment will be associated with statistically significant improvements in glucose metabolism e.g., insulin sensitivity in comparison to chronic pretreatment with olanzapine. Given the planned sample size and study duration, we hypothesize that aripiprazole treatment will be associated with numerical, but not statistically significant, improvements in comparison to pretreatment with risperidone or quetiapine. We hypothesize that aripiprazole treatment will be associated with no significant change in comparison to pretreatment with ziprasidone. These hypotheses will be evaluated by measuring insulin sensitivity and other indices via fsOGTTs and hyperinsulinemic euglycemic clamps. 