Inclusion Criteria:  Current clinically significant suicidal ideation, defined as a score of > or = 4 on the MADRS item 10 and a positive answer on items 3,4 or 5 of the C-SSRS. Inpatient status at the time of study initiation. 18 to 70 years of age Capacity to consent  Exclusion criteria:  Diagnosis of a primary psychotic disorder (e.g., schizoaffective disorder) Diagnosis of pervasive developmental disorder Diagnosis of a major neurocognitive disorder A positive urine pregnancy test Currently breastfeeding Drug or alcohol abuse or dependence within the preceding 3 months; a rather narrow time period was chosen, however, in order to allow participation by individuals with a history of substance abuse or dependence problems that could be secondary to their SI, and to more closely approximate patients seen in real-world settings. Given the increasingly widespread use of marijuana, and in an effort to recruit a naturalistic study population, concurrent marijuana use is not an exclusion criterion, as long as they are not actively intoxicated. Current positive UTOX for amphetamine, benzodiazepines, cocaine, opiates (if not prescribed) Medical issues or laboratory abnormalities requiring acute intervention Patients for whom an increase in blood pressure or intracranial pressure would pose a serious risk (e.g., aneurysmal vascular disease, arteriovenous malformation, history of intracerebral hemorrhage, unstable angina) Any lifetime history of ketamine or phencyclidine abuse A known hypersensitivity to or history of a serious adverse effect from to ketamine