For the second part of the study, you will be switched from your regular lamotrigine (Lamictal®) to once daily morning dosing of an extended-release formulation of Lamictal-XR® (at the same total daily dose). You will remain on the extended-release lamotrigine (Lamictal-XR) for two weeks, after which you will be re-admitted to the GCRC for 24 hours for Part 2 of the study. Study drug administration and blood and urine sampling will be identical to Part I. The extended-release formulation and your regular lamotrigine have been determined by the manufacturer in preliminary studies to be absorbed the same (bioequivalent0. You will not need to change your total daily dose and will only have to take your lamotrigine once daily with the extended-release product. Your blood levels of lamotrigine will be checked to insure that your are achieving the appropriate concentration with new extended-release product. For this study, you will be given a one-month supply free of charge of the extended-release formulation. After the study is completed, you would return to taking your regular lamotrigine dosing. If the FDA has approved the XR formulation at the end of the study, you and your doctor may decide to keep you on the extended-release form of the drug.