In order to explore valproate's spectrum of efficacy in rapid-cycling bipolar disorder, 55 patients underwent a prospective, open, 7.8-month trial designed to assess the drug's acute and prophylactic properties. Twenty patients received monotherapy, and 35 received combination therapy. Moderate to marked acute antidepressant responses were seen in 47% of the patients, prophylactic antidepressant responses in 76%, acute antimanic responses in 91%, prophylactic antimanic responses in 94%, acute responses in mixed states in 85%, and prophylactic responses in mixed states in 93%. Consistent with other anticonvulsant literature, these data suggest that valproate has marked antimanic and mixed state efficacy, but minimal to moderate antidepressant properties.