 Developing more effective and faster acting antidepressant is of outmost clinical importance. Available antidepressant therapies have a delayed therapeutic effect. It typically takes several weeks before symptom relief is evident. Furthermore, antidepressants are relatively ineffective - as many as 30 of patients do not respond to any medication at all. In this study the investigators evaluate the NMDA-receptor antagonist ketamine as a potentially new antidepressant treatment for severely depressed patients and compare its effectiveness with that of electroconvulsive therapy ECT. 