Decision based on strategic determination; not safety.

The purpose is to study the efficacy and safety of ABT-436 in Major Depressive Disorder.

Inclusion Criteria:

• 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.

Exclusion Criteria:

• 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.