Inclusion Criteria:
1. Male or female subjects, 18-65 years of age and body weight less than or equal to 120 kg on baseline visit.
2. Participants must have a level of understanding of the English language sufficient to agree to all tests and examinations required by the study and must be able to participate fully in the informed consent process.
3. For Healthy Controls: Subjects must be free of any lifetime psychiatric condition based on the Mini-International Neuropsychiatric Interview (MINI). For MDD: Subjects must meet Diagnostic and Statistical Manual of Mental Disorders (DSM-5) criteria for current unipolar depression \[major depressive disorder (MDD) or persistent depressive disorder (PDD) in a current major depressive episode (MDE)\] based on MINI.
4. A woman of childbearing potential who is sexually active with a male must agree to use an acceptable method of contraception \[defined as either one highly effective (permanent sterilization, intrauterine device or hormonal implant) or two other forms of contraception (such as oral contraceptive pill and condom)\] to avoid pregnancy throughout the study. Throughout the study and for 90 days (one spermatogenesis cycle) after receiving the last dose of study drug (ketamine/midazolam) man who is sexually active with a woman of childbearing potential must use an acceptable method of contraception (described above) with his female partner and must agree not to donate sperm.
5. Subjects must either be free of psychotropic medications (including antidepressants, antipsychotics, benzodiazepines, mood stabilizers, sedative/hypnotics, dopamine agonists, stimulants, buspirone, and triptans) and certain anticonvulsants (topiramate and levetiracetam) or be stable on these medications for four weeks prior to the baseline visit \[first magnetic resonance imaging (MRI) scan\].
6. Subjects with MDD should be willing to participate in neuroimaging scans before and after infusions, and be willing to undergo infusions with study drug.
Exclusion Criteria:
1. Lifetime diagnosis of schizophrenia or any psychotic disorder, bipolar disorder, pervasive developmental disorder or intellectual development disorder.
2. Current diagnosis of obsessive-compulsive disorder, anorexia nervosa or bulimia. Comorbid anxiety, stress and trauma-related disorders are permitted as long as unipolar depression is the primary diagnosis.
3. Diagnosis of a moderate or severe substance use disorder within the past 6 months per MINI; all subjects must have a negative urine toxicology test on the day of the MRI, prior to the scan.
4. Female subjects who are pregnant, nursing, for may become pregnant. Women of childbearing potential must have a negative urine pregnancy test on the day of the fMRI, prior to scan, and on days of study drug infusion, prior to infusion.
5. Any unstable medical illnesses including hepatic, renal, gastroenterologic, respiratory, cardiovascular (including ischemic heart disease), endocrinologic, immunologic, or hematologic disease.
6. Inadequately treated obstructive sleep apnea (STOP-Bang score of 5-8 if untreated, if using positive airway pressure device then past-month apnea hypopnea index ≥ 15 per hour representing moderate or higher severity).
7. Presence of a significant neurological disease such as Parkinson's disease, primary or secondary seizure disorders, intracranial tumors, or severe head trauma.
8. Presence of neurocognitive or dementing disorders.
9. Clinically significant abnormalities of laboratories, physical examination (including unstable hypertension - systolic blood pressure \>170, diastolic blood pressure \>100), or electrocardiogram at screening visit.
10. Subjects judged to be at serious and imminent suicidal or homicidal risk by the PI or another study-affiliated psychiatrist.
11. Any contraindications to MRI, including pacemakers or metallic objects in the body.
12. Any claustrophobia or other conditions which may result in inability to lie still in the MRI scanner for 1 hour or more.
13. Allergy to ketamine or midazolam in subjects with MDD.
14. Must not be on any prohibited concomitant medication.