Patients with bipolar I disorder (BD) experience depression 3 times more frequently than mania, and antidepressants are prescribed as adjuncts to mood stabilizers in up to 70% of patients. However, no placebo-controlled trials have assessed the efficacy or safety of modern antidepressants in combination with mood stabilizers in the maintenance treatment of BD. The investigators propose a multicentre, randomized, double-blind clinical trial comparing mood stabilizer plus antidepressant (escitalopram or bupropion XL) to mood stabilizer plus placebo in the maintenance treatment of BD.