 The primary objective of the study is to assess the efficacy and tolerability of a 12-week trial of memantine hydrochloride administered twice daily in 20 adults ages 18-55 with ADHD and ADHD NOS. Improvement will be defined as: 1 changes from baseline on the investigator-rated DSM-IV based ADHD Rating Scale; 2 changes from baseline in a questionnaire aimed at assessing executive functions BRIEF; and 3 changes from screening in a computerized neuropsychological battery CANTAB. We hypothesize that memantine hydrochloride will be associated with improving ADHD symptoms and associated deficits in executive functions. We also expect that memantine will be well-tolerated with predictable adverse events. 