This study has two phases. The first phase is the washout phase that will last for 7 days. The second phase is an 8-week acute treatment phase in which the efficacy and tolerability of felbamate and placebo are compared. Lithium can remain during Study Periods I and II if partial response to this agent is documented. Patients who complete the 8-week double-blind phase will receive clinical treatment. Acute efficacy will be determined by demonstrating a greater response rate using specified criteria.