Patients diagnosed with mood disorders and healthy volunteers will receive positron emission tomographic (PET) scans with H215O while doing simple tasks. Patients will continue to receive scans while in different mood states and while taking different medications. Patients eligible for this study will be participating in other research studies measuring other clinical and biochemical parameters (mood and anxiety ratings, medication responses, and psychological test results). Information gathered from H215O PET scans measuring blood flow to specific brain areas will be compared to the data gathered from other studies.