Participation in this double-blind study will last up to 18 weeks. Participants will be randomly assigned to receive either lithium, sertraline, or lithium and sertraline. Both medications will initially be given at a low dose and then gradually increased over 2 weeks. For the remainder of the study, dosages will be adjusted as necessary. Study visits will occur every week for the first 6 weeks and then every other week for the remaining 10 weeks. During all study visits, participants will complete a psychiatric assessment and questionnaires about their current mood and any treatment side effects. Urine and blood collection may occur at selected times during the study.