The occurrence of a new manic/depressive episode was considered a treatment failure, and the patient was discontinued from the study. At the end of Part II, 6 months after last patient enrolled and after no longer than approximately 15 months, if patients were still in remission and the investigational product was well-tolerated, patients had the option to enter long-term open-label treatment at the same dosage as used in Part II until a new episode occurred, until marketing was authorized, or until clinical development of BIA 2-093 in the recurrence prevention indication was discontinued. If patients did not enter long-term treatment, an established recurrence prevention medication was prescribed, and BIA 2-093 was tapered off (patients assigned to 1800 mg had the daily dose decreased to 900 mg for 6 days; those assigned to 900 mg or 300 mg received placebo for 6 days).