We aim to determine why patients with depression are at an elevated risk for the development of coronary heart disease, and resolve whether the severity of a patient's depression has a counterpart in demonstrable abnormalities in brain chemistry. Studies will be completed in 28 patients with depression; both males and females. Patients will be studied both untreated and during administration of a selective serotonin re-uptake inhibitor (SSRI) antidepressant. They will be either newly diagnosed with depression, untreated patients suffering a recent relapse, or patients seeking to switch from a non-SSRI antidepressant due to non-response. The turnover of chemical messengers in the brain will be estimated by high internal jugular venous blood sampling and DNA will be isolated and examined from blood cells. Immune function will also be assessed. Whole body and cardiac sympathetic nervous activity will be determined, as well as microneurographic recording of muscle sympathetic nervous activity.