OBJECTIVE: The authors compared the new extended-release and standard preparations of divalproex sodium. METHOD: Twelve patients with DSM-IV bipolar disorder or schizoaffective disorder who were clinically stable while taking the standard form of divalproex participated in the study. These patients were given a single daily dose of the extended-release preparation of divalproex in an open 6-week trial. Clinical symptoms and adverse effects were rated weekly. Doses were adjusted to maintain steady serum valproate concentrations. RESULTS: The medication change was associated with negligible changes in clinical status and tolerability. To maintain serum drug levels, however, 21% higher doses of the extended-release preparation were required. CONCLUSIONS: Use of extended-release divalproex once a day was as well tolerated as the standard preparation, with no change in efficacy within 6 weeks, but the daily dose needed to maintain stable serum valproic acid concentration was 21% higher.