Participants in this study will be admitted to the Clinical Center for up to 10 weeks. At study entry, participants will have a 7-day washout period in which they will be tapered off all psychiatric medications, with the possible exception of lithium, and will be given a placebo (an inactive pill). After the washout period, participants will be randomly assigned to receive either felbamate or placebo for 8 weeks. Participants whose depression symptoms worsen by more than 30% or those for whom study continuation is considered potentially harmful will be taken off the study and offered open-label treatment. Participants who received felbamate and responded well to treatment will have the option of continuing treatment.