The purpose of this study is to evaluate the effectiveness, safety and tolerability of an escitalopram combination treatment compared to single treatments, and to placebo in patients with major depressive disorder.

Both active drugs being tested, alone and in combination, in this study are currently available antidepressants. However, the doses used in this study are lower than the standard approved doses.

Inclusion Criteria:

• Patients must meet Diagnostic and Statistical Manual, Fourth Edition (DSM-IV) diagnostic criteria for Major Depressive Disorder
• The patient's current depressive episode must be at least 12 weeks in duration.

Exclusion Criteria:

• Women who are pregnant, women who will be breastfeeding during the study, and women of childbearing potential who are not practicing a reliable method of birth control.
• Patients who currently meet DSM-IV criteria for : a. bipolar disorder; b. schizophrenia or any psychotic disorder; c. obsessive-compulsive disorder; d. mental retardation or any pervasive developmental disorder or cognitive disorder.
• Patients who are considered a suicide risk.
• Patients with a history of seizure disorder, or any history of seizure, stroke, significant head injury, or any other condition that predisposes toward risk of seizure.