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. The study is a multicenter, US and Canadian, randomized, double-blind, 3-parallel-group, placebo- and escitalopram-controlled, Phase III study consisting of four segments (A, B, C and D). Segment A is a 1-week, placebo, single-blind period. Segment B is an 8-week, double-blind period. Segment C is an optional 18-week double-blind extension period. Segment D is a 1-week safety follow-up period after study drug discontinuation or early termination (during Segment B or C).