Aripiprazole has been approved by the FDA for augmenting ineffective/partially effective oral antidepressant therapy in patients suffering from major depression. The mechanism by which this augmentation is achieved is not known. This study has been designed to test the hypothesis that the primary mechanism of action of aripiprazole antidepressant augmentation through the dopaminergic pathway. Two positron emission tomography (PET) scan procedures and a functional magnetic resonance imaging (MRI) scan will be used to test this hypothesis.