This study will report on two related trials. In the outpatient trial, patients will be administered adjunctive ketamine at two different doses (0.25mg/kg; 0.5mg/kg), and a placebo (saline), across 3 consecutive sessions within their regular maintenance ECT course. The order of conditions will be randomised across participants. Patients will be required to learn some words and faces 20 minutes prior to ECT, and complete a detailed cognitive battery 4 hours after ECT on each of the 3 occasions. The purpose of this trial is to determine whether ketamine is superior to placebo in reducing cognitive impairment following ECT and what the optimal dose of ketamine is for minimising cognitive and other side effects. Projected sample for this trial is N = 17.