Serotonin (5-HT) is important in mood regulation and is believed to play a major role in the pathophysiology of major depression (MD). Selective serotonin reuptake inhibitors (SSRIs) are currently the most widely used drugs for the treatment of depression. Patients with bipolar disorder (BD) who are treated for depression with SSRIs might develop mania, which is believed to be triggered by antidepressant treatment. Rapid tryptophan depletion (RTD) has been shown to induce transient depressive symptoms in remitted depressed patients treated with SSRIs. In remitted manic patients treated with lithium, RTD does not seem to have clinical effects. However, RTD was not studied in acutely manic patients and could theoretically have antimanic properties. In this double blind randomized placebo controlled study RTD will be used as a tool to assess the role of 5-HT in mania and will be evaluated for its antimanic properties and potential use as augmentation to drug treatment in acute mania.