The specific aim of this study is to evaluate the efficacy, tolerability, and safety of divalproex extended release (DPX-ER) monotherapy in the treatment of ambulatory bipolar spectrum disorder with moderate-to-severe hypomania or mild mania in a randomized, double-blind, placebo-controlled, parallel group, 8-week trial in 60 outpatients with bipolar disorder, types I, II, or NOS by DSM-IV-TR criteria. Patients may have concurrent depressive and anxiety symptoms and syndromes, but not psychotic features or substance dependence. The primary outcome measure will be change in hypomanic/manic symptoms from baseline to endpoint. Secondary outcome measures will include change in depressive and anxiety symptoms, change in global severity of illness, concomitant use of adjunctive rescue anxiolytic and hypnotic medications, adherence with study medication, and degree and severity of side effects experienced.