Bipolar disorder (BPD) is a common, recurrent, and disabling medical condition. Although mania is the defining feature of BPD, depression represents the majority of illness burden in patients with this devastating condition. Despite the high degree of morbidity and mortality associated with bipolar depression, currently available treatments are few and often inadequate. Recently, a single intravenous (IV) dose of the N-methyl-D-aspartate (NMDA) glutamate receptor antagonist ketamine has demonstrated rapid antidepressant effects in severe unipolar depression. Therefore, the objective of the current study is to investigate the safety and efficacy of a single IV dose of ketamine in treatment-resistant bipolar depression (TRBD).