Major depressive disorder is one of the most important psychiatric issues worldwide, with important prevalence of treatment-resistant depression (TRD). Non-monoaminergic agents are currently in the spotlight. Objective was to explore for information about mechanisms of action of ketamine, its connections with copper and possible importance for TRD treatment. There are at least few possible pathways for ketamine action in depression in which copper and other divalent ions may show a vital role. There is urgent need for more studies to gather information about correlation between ketamine, copper and antidepressive features of these agents.