Additional Study Information: Overview of the Research Study: This study uses brain-scanning technology (MRI) to help understand actions of an experimental antidepressant called ketamine, which is used widely in higher doses for anesthesia. In this study, we seek to determine whether HNKs (which are made from ketamine in the body) are responsible for ketamine's antidepressant effect. If there is any benefit, it is expected to appear within 24 hours. Procedure: You will be given a single dose, intravenously (I.V.), of an experimental antidepressant called ketamine, while being scanned by an MRI to measure the chemical effect of the drug. Ketamine will be given in a dose of 0.3, 0.5, or 0.7 mg/kg. Patients enrolled in the study are eligible for up to 6 months of treatment with their study psychiatrist after the ketamine infusion. During this time, patients will be responsible for the cost of the conventional antidepressants, but all doctors visits will be free of charge. Benefits to the Participant: If eligible, you will receive: a health evaluation and up to six months of outpatient psychiatric treatment at the New York State Psychiatric Institute at no cost to you, and up to $100 compensation. Each eligible participant receives the following services at no cost: A thorough clinical evaluation by a doctoral-level psychologist and/or psychiatrist to properly diagnose your mood disorder; a medical evaluation and assessment of thyroid, liver and kidney function, electrolytes, blood count, and cholesterol; a total of six months of treatment with a doctor at Columbia University Medical Center; and up to $100 in compensation for your time and effort. There is no certainty that the ketamine will help your depression.