The new atypical antipsychotic drugs have proven to be very effective in the treatment of psychoses, however, one very alarming side effect of these drugs is excessive weight gain. In humans it has been shown that average weight gains of 4 to 4.5 kg can occur during a 10-week treatment period to nearly a 12 kg increase after one year of treatment. This increase in body weight is associated with an increase in impaired glucose tolerance and hypertension and therefore is likely to increase mortality rates. The mechanisms involved in the drug-induced weight gain are currently unknown. We have developed a mouse model in which we can induce weight gain with three of the currently available and commonly prescribed antipsychotic drugs (olanzapine, quetiapine, and risperidone). Weight gain in our model is reproducible and occurs within four weeks using twice-daily oral treatment. We hypothesize that olanzapine and quetiapine produce weight gain via increased food intake and risperidone produces weight gain by decreasing energy expenditure. We further hypothesize that these drug-induced changes in food intake and energy expenditure are due to differential changes in hypothalamic gene expression patterns relating to alternative mechanisms of regulating energy balance. Lastly, alterations either in food intake, body weight, and/or body composition, or the direct action of the drugs will produce decreases in insulin sensitivity in vivo. With the proposed studies, we will determine the mechanisms of drug-induced weight gain associated with these drugs. Once known, attempts could be made to avoid the deleterious side effects, or at least allow one to more accurately consider benefits versus risks dependent upon the presence of other confounding variables (family history of obesity, diabetes, and hypertension).