This is a single center, eight-week, randomized, double-blind, placebo-controlled, parallel group, flexible-dose study in 40 outpatients with bipolar disorder types I, II, or NOS by DSM-IV-TR criteria who also have moderate-to-severe hypomanic symptoms or mild manic symptoms (defined as a score of > 3 but <5 on the Clinical Global Impressions Scale Modified for Bipolar Disorder [CGI-BP]). Subjects may have concurrent depressive or anxiety symptoms or syndromes, but they may not have psychotic features (as defined in DSM-IV) or substance dependence. The primary outcome measure will be change in the Young Mania Rating Scale (YMRS) scores. Secondary outcome measures will include the total of the YMRS and the Inventory for Depressive Symptoms (IDS), the IDS alone, the Hamilton Rating Scale for Anxiety (HAM-A), the CGI-BP, the Global Assessment of Functioning (GAF), the Life Functioning Questionnaire (LFQ), and the Sheehan Disability Scale (DSD). Of note, the YMRS-IDS total reflects a novel means of assessing total affective burden. Also, IDS will be used to assess depressive symptoms because of our experience in the Stanley Foundation Bipolar Network indicating that this scale better assesses the signs and symptoms of bipolar depression as compared to both the Hamilton Rating Scale for Depression (HAM-D) and the Montgomery-Asberg Depression Scale (MADRAS). The LFQ is a validated gender neutral scale specifically designed to assess functional outcome in bipolar disorder. The Abnormal Involuntary Movement Scale (AIMS), Simpson Angus Scale (SAS), and Barnes Akathisia Scale (BARS), will be used to assess for degree and severity of extrapyramidal side effects experienced.