Patient description: A 46-year-old Asian woman with MDD complains of persistent feelings of sadness and loss of interest in daily activities. She states that her mood is still depressed most of the days. She complains of loss of energy and feelings of worthlessness nearly every day. She is on anti-depressants for the past 6 months, but the symptoms are still present. She does not drink alcohol or smoke. She used to exercise every day for at least 30 min., but she doesn't have enough energy to do so for the past 3 weeks. She also has some digestive issues recently. She is married and has 4 children. She is menopausal. Her husband was diagnosed with colon cancer a year ago and is undergoing chemotherapy. Her past medical history is unremarkable. Her family history is negative for any psychologic problems. Her HAM-D score is 20.

Clinical trial eligibility criteria: eligible ages (years): 18.0-55.0,  Major Depressive Disorder Unipolar, major depressive episode (MDE), with 17-item Hamilton Depression Rating Scale score ≥16. Patients may be psychiatric medication free, or if currently taking psychiatric medication, not responding adequately as evidenced by current MDE 55 years old Female patients of child-bearing potential must be willing to use an acceptable form of birth control during study participation such as condoms, diaphragm, oral contraceptive pills Must be enrolled in division's umbrella research protocol Able to provide informed consent Agrees to voluntary admission to an inpatient research unit at The New York State Psychiatric Institute (NYSPI) for baseline PET imaging and Magnetic Resonance Imaging (MRI), ketamine infusion, and repeat PET imaging  Unstable medical or neurological illness including: A) baseline hypertension (BP>140/90); B) significant history of cardiovascular illness; C) Platelet count < 80,000 cells/uL; and D) Hemoglobin < 11 g/dL for females and < 12 g/dL for males Significant electrocardiogram (ECG) abnormality (e.g., Ventricular tachycardia, evidence of myocardial ischemia, symptomatic bradycardia, unstable tachycardia, second degree (or greater) atrioventricular (AV) block) Pregnancy, currently lactating, or planning to conceive during the course of study participation Diagnosis of bipolar disorder or current psychotic symptoms Current or past ketamine use disorder (lifetime); any drug or alcohol use disorder within past 6 months Inadequate understanding of English Prior ineffective trial of or adverse reaction to ketamine A neurological disease or prior head trauma with evidence of cognitive impairment. Subjects who endorse a history of prior head trauma and score ≥ 1.5 standard deviations below the mean on the Trailmaking A&B will be excluded from study participation Metal implants or paramagnetic objects contained within the body (including heart pacemaker, shrapnel, or surgical prostheses) which may present a risk to the subject or interfere with the MRI scan, according to the guidelines set forth in the following reference book commonly used by neuroradiologists: "Guide to MR procedures and metallic objects," F.G. Shellock, Lippincott Williams and Wilkins NY 2001. Additionally transdermal patches will be removed during the MR study at the discretion of the investigator Current, past, or anticipated exposure to radiation, that may **