Ketamine is a potent antagonist of the N-methyl-D-aspartate (NMDA) receptor, a major type of glutamate receptor in the brain. In a placebo-controlled study completed at Yale a single dose of ketamine (0.5 mg/kg, intravenously) had rapid antidepressant effects in depressed patients. In these subjects ketamine infusion produced mild psychotomimetic symptoms and euphoria that dissipated within 120 minutes, while the antidepressant effects of ketamine infusion emerged over the first 180 minutes and persisted over 72 hours. Fifty percent of depressed patients receiving ketamine were treatment responders at Day 3 compared to 12.5% in the placebo infusion group. These results have been replicated in a recent double-blind study performed at NIMH and a third unpublished study conducted by members of our group at Yale.