Alzheimer amyloid β-peptide (Aβ) is a physiological peptide constantly anabolized and catabolized under normal conditions. We investigated the mechanism of catabolism by tracing multiple-radiolabeled synthetic peptide injected into rat hippocampus. The Aβ 1–42 peptide underwent full degradation through limited proteolysis conducted by neutral endopeptidase (NEP) similar or identical to neprilysin as biochemically analyzed. Consistently, NEP inhibitor infusion resulted in both biochemical and pathological deposition of endogenous Aβ 42 in brain. This NEP-catalyzed proteolysis therefore limits the rate of Aβ 42 catabolism, up-regulation of which could reduce the risk of developing Alzheimer's disease by preventing Aβ accumulation.