Ketamine treatment represents a potentially viable, safe and effective treatment for MDD/bipolar depression + SI. The Investigators therefore propose to conduct a pilot trial in preparation for a full- scale randomized controlled trial (RCT) which would aim to determine the efficacy of IV ketamine + a standard medication treatment (Treatment As Usual; TAU) vs. midazolam, an "active" placebo + TAU in treating SI among inpatients with MDD and in inpatients with bipolar depression. If the full-scale RCT demonstrates ketamine's efficacy, it would have important implications for both future research as well as inpatient treatment.