Accordingly, we will be test whether a 100-hour ketamine infusion would be more effective than the standard 40-minute ketamine infusion currently used in other TRD studies. We will randomize subjects to one of 2 arms: (1) 100-hour (+/- 4 hours) ketamine infusion plus clonidine for the entire infusion (2) 40-minute ketamine infusion (plus clonidine) following a 100+/- hour saline infusion. All subjects will receive clonidine, an alpha-2 agonist, to minimize side effects of ketamine (namely, brief/mild psychotic and cognitive symptoms).