DEPOSITS of β-amyloid are apparent in ageing and Alzheimer's disease¹, but the role of this peptide in neurodegeneration is unclear². The free-radical theory of ageing may also account for Alzheimer-type degeneration and consequently links between free-radical generation and β-amyloid have been sought³. We demonstrate here that β-amyloid interacts with endothelial cells on blood vessels to produce an excess of superoxide radicals, with attendant alterations in endothelial structure and function. The superoxide radical can scavenge endothelium-derived relaxing factor and produce potent oxidizing agents, which can cause lipid peroxidation and other degenerative changes⁴. The alterations in vascular tone and endothelial damage are prevented by the oxygen-radical-scavenging enzyme superoxide dismutase. These observations suggest a normal vasoactive role for β-amyloid as well as a mechanism by which β-amyloid may play a role in vascular abnormalities and neurodegeneration mediated by free radicals.