The study is designed to evaluate the safety and efficacy of fluoxetine for treating children and adolescents with Bipolar Disorder who are experiencing an episode of major depression while being treated with a mood stabilizer. The study involves a 2-week assessment period. Patients who are on stable, therapeutic doses of lithium or valproate and continue to have depression will be randomized to a 12-week treatment of fluoxetine or placebo. Those who respond favorably to treatment will be followed openly for an 18-week continuation phase.