 Ketamine KET has been used for decades as a sedative and anesthetic. In treatment-resistant depressed patientsTRD, an intravenous dose much lower than necessary for anesthesia may produce a robust antidepressant effect and may even abolish suicidal thoughts within hours, peaking within 24 hours. But, its antidepressant effect generally lasts only days.Previous studies examining KET in TRD have been critiqued for lack of an effective placebo measure due to brief perceptual experiences associated with KET. Thus, the current study compares KET against a short-acting sedative. The phases of this study compare response to a single KET injection to 6 injections over 2 weeks. Next, KET responders are given 1 injection a week for 3 weeks of either KET or the sedative agent to determine if beneficial effects of KET are maintained, and to assess duration of its benefits after repeated administration. The genetic profile of patients for a substance promoting contacts between cells and brain will be determined to investigate if response to KET could be predicted with that blood test. This substance, as well as several chemicals that produce inflammation, will also be measured in the blood to investigate their role in the effect of KET. Patients will receive, in total, no more than the equivalent of two to three anesthetic dose of KET. Results from this study will help establish the beneficial effects of a single KET injection as a rapid intervention for major depression, and to investigate the possibility of obtaining a prolonged antidepressant effect with repeated injections. 