Major Depressive Disorder (MDD) is a disabling medical illness and current monoaminergic treatments are slow to act and possess only limited efficacy. In this context, the discovery that the glutamate NMDA receptor antagonist ketamine is rapidly antidepressant (onset of action within hours) -- even in patients suffering from treatment-resistant depression (TRD) -- has ignited tremendous enthusiasm among clinicians, scientists and patients alike. A critical obstacle to the translation of this discovery into a novel treatment, however, is the limited duration of action following a course of ketamine (e.g. 1-2 weeks). The current project will address this important gap in medical knowledge by testing a rational neuropharmacological strategy designed to optimize and sustain the rapid antidepressant effects of ketamine. Driven by the recent characterization of the molecular mechanisms underpinning the antidepressant and neuroplasticity effects of ketamine, we will test the combination of ketamine plus lithium in patients with TRD using a randomized, double blind, placebo-controlled design. The primary aims of the project are (1) to test the efficacy of lithium-plus-ketamine compared to placebo-plus-ketamine as an antidepressant combination strategy in TRD and (2) to gather data on the safety and tolerability of the lithium-plus-ketamine.