This randomized, double-blind, placebo-controlled, parallel group study will evaluate the safety and efficacy of RO4917523 as adjunctive therapy in patients with major depressive disorder and an inadequate response to ongoing antidepressant therapy. Anticipated time on study treatment is 6 weeks with a 3-week follow-up.

Inclusion Criteria:

• Adult patient, 18 to 70 years of age at time of informed consent
• Major depressive disorder without psychotic features as defined by DSM-IV-TR criteria
• Inadequate response to ongoing antidepressant treatment, as defined by protocol
• Body mass index (BMI) 18 to 38 kg/m2 inclusive

Exclusion Criteria:

• Currently receiving treatment with a combination of antidepressants (two or more), or an adjunctive potentiating treatment as defined by protocol
• Previously received RO4917523
• History of failure, or utilization during the current episode of Electroconvulsive Therapy (ECT) or repetitive Transcranial Magnetic Stimulation (rTMS)
• History of use at any time of Vagus Nerve Stimulation (VNS) or Deep Brain Stimulation (DBS)
• Current or past history of bipolar disorder (e.g. manic, hypomanic, or mixed episodes)
• Pregnant or lactating women