At baseline (Visit 1), subjects whose screening evaluations continue to meet all inclusion/exclusion criteria may enter the study and be randomized. Baseline ratings will be obtained, including the YMRS, IDS, HAM-A, and the Clinical Global Impression (CGI-BP). The subject must be evaluated as having modest-to-severe hypomania or mild mania on at least 2 occasions at least 3 days apart to be entered into the randomized phase. Also, the subject's bipolar disorder must be evaluated as mildly to moderately severe on at least 2 occasions at least 3 days apart, defined as a CGI-BP of > 2 and < 5, for the patient to be entered into the randomized phase. Subjects will also be evaluated with the Inventory for Depressive Symptoms (IDS), Hamilton Scale for Anxiety (HAM-A), Global Assessment of Functioning Scale - clinician rated (GAF-C), and the Sheehan Disability Scale (SDS). Blood pressure, pulse, height, and weight will be measured. Study medication will be dispensed by the Physician Investigator in the form of 500mg capsules. The dose starting range of divalproex ER will be 15 mg/kg/day.