The roles of metal ions both on the formation of fibril aggregation of amyloid-β-peptide (Aβ) and on the generation of superoxide by a metal-Aβ complex have been studied to clarify the relationship of both phenomena with the cause of Alzheimer's Disease (AD). Among the several metal ions, the addition of Zn(II) or of Cu(II) was found to increase the formation of Aβ fibril. Although Zn(II) promoted the Aβ fibril formation both at neutral and acidic pH levels, Cu(II) could induce the aggregation of Aβ only at acidic pH. In addition, hydrogen peroxide was generated at neutral pH in the absence of Aβ and Cu(II) beyond a stoichiometric amount of Cu(II) ion, suggesting the formation of a catalytic site that can react between Cu(II) and Cu(I). These results indicate that Zn(II) and Cu(II) at different pHs may each contribute to AD in different modes; the promotion of fibril aggregation (Zn(II) at every pH and Cu(II) at acidic pH) and the formation of superoxide (Cu(II) at neutral pH). The most characteristic aspect is the effect of pH on the promotion of aggregation and the formation of superoxide for the Cu(II)-Aβ system. The coordination structure of the Cu(II)-Aβ complex at neutral and acidic pHs was analyzed by using NMR spectroscopy, circular dichroism (CD), and ESR. The formation of a complex of histidine (His) residues and Cu(II) was found, and the coordination number of the His was suggested to be two at neutral pH and three at acidic pH. These results imply that the intramolecular coordination structure of the Aβ -Cu(II) complex at neutral pH could be converted to the intermolecular one by acidification. The pH variation thus led to a change in the complex structure of Cu(II)-Aβ. This finding suggests the important roles of pH to regulate two phenomena, i.e. the aggregation of Aβ (inhibition of superoxide formation) and the formation of superoxide from Cu(II)-Aβ complex.