An in vitro effect of (+)­MK-801 (dizocilpine), an inhibitor of the glutamate/NMDA and nicotinic acetylcholine receptors, on the Aβ1–42 and Aβ1–40 peptides is described and compared to that of memantine. Memantine has been approved by the U.S. Food and Drug Administration for the treatment of mild-moderate Alzheimer’s disease. Both compounds accelerated the formation of a β-sheet structure by Aβ1–42, (+)­MK-801 more rapidly than memantine, as observed in a thioflavin T fluorescence assay. The acceleration was followed by a decrease in the fluorescence signal that was not observed when the ligand was absent. Nuclear magnetic resonance spectra of the soluble peptides in the presence and absence of (+)­MK-801 demonstrated that the monomeric form did not bind (+)­MK-801 and that in the presence of (+)­MK-801 the concentration of the monomeric form progressively decreased. Small angle X-ray scattering confirmed that the presence of (+)­MK-801 resulted in a more rapid and characteristic transition to an insoluble form. These results suggest that (+)­MK-801 and memantine accelerate the transition of Aβ1–42 and Aβ­1–40 to ThT-negative insoluble forms.