Memantine-d6 is intended for use as an internal standard for the quantification of memantine by GC- or LC-MS. Memantine is an NMDA receptor antagonist that blocks NMDA-induced currents in rat retinal ganglion cells by 90% when used at a concentration of 12 μM. It reverses inhibition of dephosphorylation of the synthetic tau phosphopeptide p17 (tau194-207) induced by the endogenous inhibitor of protein phosphatase 2A (PP2A) I1PP2A in vitro. In vivo, memantine (2 mg/kg) restores PP2A activity, decreases GSK-3β and amyloid-β (Aβ) levels in the hippocampus, cerebral cortex, and ventricular areas, and attenuates spatial learning and memory deficits in the AAV1-I1PP2A rat model of Alzheimer's disease. Memantine (20 mg/kg) reduces responding on the ethanol-associated lever in a cue-induced ethanol-seeking test in rats. It also decreases secretion of matrix metalloproteinase-9 (MMP-9), degradation of collagen IV, the size of cerebral ischemia-induced brain infarcts, and neuronal cell death in a mouse model of focal cerebral ischemia.