This multi-center, randomized, double-blind, placebo-controlled, parallel-group study will investigate the efficacy and safety of RO4995819 as adjunctive therapy in patients with major depressive disorder having inadequate response to ongoing antidepressant treatment. Patients will be randomized to receive once daily doses of 5 mg, 15 or 30 mg of RO4995819 or matching placebo. The anticipated time on study treatment is 6 weeks.