Metabolism of amyloid-beta peptide (A beta) is closely associated with the pathology and etiology of Alzheimer's disease (AD). Neprilysin is the only rate-limiting catabolic peptidase proven by means of reverse genetics to participate in A beta metabolism in vivo. The aim of the present study is to assess whether possible spatial changes in neprilysin level in the brain with aging correlate to AD-vulnerable regions. When neprilysin levels in various brain regions of 10-, 80- and 132-week-old mice were evaluated by neprilysin-dependent endopeptidase activity assay and Western blot-based quantitative analysis, a clear change in neprilysin level with aging was observed in the hippocampal formation, in which the level was reduced by 20% at 132 weeks, compared to the 10-week group. Quantitative immunohistochemical analysis confirmed a marked local reduction of neprilysin levels with aging at the outer molecular layer and polymorphic layer of the dentate gyrus, and the stratum lucidum of the hippocampus, where the densities were reduced by 56%, 82% and 83%, respectively, at 132 weeks compared to the 10-week group. Thus, neprilysin was decreased selectively at the terminal zones and on axons of the lateral perforant path and the mossy fibers. These are the sites that show A beta pathology in mutant amyloid precursor protein (APP) transgenic mice, and that show synaptic loss in AD. The immunoreactivities to synaptic vesicle protein-2 and synaptophysin in the stratum lucidum and the dentate gyrus were unchanged, suggesting that a loss or decrease of synapses was not responsible for the decrease in the neprilysin levels. These observations suggest that downregulation of neprilysin is likely to be related to AD pathology and to the A beta deposition associated with normal aging in humans.