 Memantine is an NMDA receptor antagonist that improves cognitive and behavioural deficits in patients with Alzheimer disease, vascular dementia and mixed dementia. This study is focused in proving the efficacy of Memantine in ameliorating one of the most frequent symptoms of patients with MS which is attention and memory deficits. Memantine is a safe drug in patients with MS and it has been administered to MS patients with pendular nystagmus Starck et al J Neurol 1997. The study will have the power to detect differences in such clinical question by studying 60 MS patients with cognitive impairment n60 with a crossover design. Indeed, we plan to use a new and powerful surrogate marker such as attention evoked potentials developed in our center. Finally, because there are evidences that Memantine might improve MS outcome by closing the Brain-Blood barrier which is the best therapeutic target in this disease Paul et al J Pharmacol Exp Ther 2002, an exploratory study of its efficacy in preventing new MRI lesions might also be included in the design. 