Efficacy and Safety Study of ABT-436 in Major Depressive Disorder

* Has a primary Diagnostic and Statistical Manual of Mental Disorders-Fourth Edition-Text Revision diagnosis of major depressive disorder without psychotic features * No current antidepressant treatment within 7 half-lives or 2 weeks, whichever is longer, prior to Day minus 1. * Can safely be treated on an outpatient basis. * A condition of general good physical health. * Surgically sterile, using a highly effective method of birth control or (if female) at least 1 year post menopausal.

* History of hypersensitivity, intolerance or adverse reaction to escitalopram that led to discontinuation, or hypersensitivity to citalopram. History of serotonin syndrome. * Inadequate response to more than two different antidepressant medications during the current major depressive episode. * History of electroconvulsive therapy, vagal nerve stimulation or deep brain stimulation. * History of transcranial magnetic stimulation during the current major depressive episode. * Psychotherapy that has not been ongoing for at least 3 months prior to Day minus 1.