This will be a rater-blinded, head-to-head comparison (no placebo) of divalproex ER and quetiapine in patients with symptoms of an active manic or mixed mania (symptoms of mania and depression). Forty subjects are expected to be enrolled. After screening for eligibility, eligible subjects will be randomized while hospitalized in a 1:1 ratio into 2 treatment groups: divalproex ER or quetiapine. Depakote® ER will be given orally at 30 mg/kg day initially taken at night and rounded up to nearest 500 mg dose with adjustments made through the trial as needed to obtain serum valproic acid levels of 85-125 mcg/ml. Quetiapine will be given orally at an initial dose of 200mg/day on Day 1, and titrate up to 800 mg/day. The duration of the study will be 21 days from baseline and the total number of visits including screening is five. Patients will be released from the hospital once stable and visits for the study will then take place on an outpatient basis.