We are studying subjects with mild to moderate Alzheimer's disease who have been on a stable dose of any cholinesterase inhibitor [donepezil (Aricept), rivastigmine (Exelon), or galantamine (Razadyne)] for at least 3 months, and have not previously taken memantine (Namenda). This is an open-label study, with magnetic resonance spectroscopy (MRS) as the primary outcome measure, along with neuropsychological testing, and optional lumbar puncture, evaluating patients on their stable dose of a cholinesterase inhibitor over 24 weeks, followed by another 24 weeks on memantine in combination with stable dose of cholinesterase inhibitor. The purpose of this study is to characterize the progression of disease using MRS, cerebrospinal fluid (CSF) biomarkers, and cognitive outcome measures, and to determine whether changes in cognitive function on neuropsychological testing are correlated to changes in MR spectroscopic and/or CSF biomarkers.