Twenty subjects with clinically definite MS and objectively documented new learning impairment will be included in the study. All subjects will undergo baseline neuropsychological testing and EDSS to document current levels of functioning in new learning and memory abilities. Subjects will then be randomly assigned to either group 1 or group 2. Group 1 (n=10) will first undergo treatment with Modafinil (200 mg once per day in the morning) for 2 weeks. They will then undergo follow-up neuropsychological assessment and follow-up EDSS to evaluate any medication effects. After the follow-up evaluation, there will be a washout period of one week in which no medication will be administered. Group 1 will then receive a placebo medication for 2 weeks. A second follow-up evaluation will be conducted following this latter arm of the study. Group 2 (n=10) will follow the same pattern, but will receive the placebo medication during the first arm of the study and Modafinil during the 2nd arm of the study. The subjects and the experimenter administering the neuropsychological evaluation will be blind to group membership.