The subjects will be randomized to a crossover trial of memantine and placebo for 8 weeks followed by a 4 week washout and then 8 more weeks of study drug. Memantine will be started at 5 mg/day for 1 week, increased to 5 mg BID in the second week, 10 mg QAM and 5 mg QHS the next week and to 10 mg BID in weeks 4-8 as directed in the package insert. One 8 week course will be memantine and the other placebo assigned in a random fashion. Measures of memory will be compared within subjects at baseline, week 4, week 8, week 12 (beginning of second course of study drug), week 16 and week 20 (exit). At weeks 2 and 14, participants will be given 3 questionnaires that will measure mood; medication management will be conducted during these assessments. Each visit, including baseline, should take approximately one and a half hours.