To evaluate the efficacy of a fixed dose of SR58611A(350 mg q12) compared to placebo in patients with MajorDepressive Disorder (MDD) using escitalopram (10 mgqd) as positive control. In addition, the tolerability and safety of SR58611A in patients with MDD will be evaluated. The present study is an 8-week, double-blind, placebo- and escitalopram-controlled, randomized, parallel-group study. A 1-week, placebo, single-blind period precedes the 8-week randomized treatment period and an optional 18-week, double-blind extension period follows the randomized treatment period. A Safety Follow up Visit is scheduled 1 week after the acute and extension period, or early termination. Escitalopram, a selective serotonin reuptake inhibitor (SSRI), an approved treatment for MDD, is chosen as a positive control agent in this study. The dose of 10 mg is within the approved dose range with no need for dose adjustment in elderly patients. This trial is designed to formally compare the efficacy, safety, and tolerability of SR58611A to placebo. Escitalopram is used as a positive control.