 This will be a 12-week, open-label pilot study to assess the efficacy and tolerability of memantine hydrochloride Namenda administered to 20 adults 18-55 years of age with ADHD and ADHD NOS. All subjects that enter the study will undergo standard screening and diagnostic procedures. After obtaining written informed consent from the subject, the diagnosis of ADHD will be established through clinical evaluation by an expert clinician. Only consenting subjects satisfying inclusion and exclusion criteria will be included in the study. 