Cerebral amyloid angiopathy (CAA) is common in Alzheimer's disease (AD) and may contribute to dementia and cerebral hemorrhage. Parenchymal β-amyloid deposition is dependent on the activity of zinc transporter 3 (ZnT3), a neocortical synaptic vesicle membrane protein that causes enrichment of exchangeable Zn2+ in the vesicle, which is externalized on neurotransmission. However, the contribution of zinc to vascular β-amyloid deposition remains unclear. Here, we identify for the first time an exchangeable pool of Zn2+ in the cerebrovascular wall of normal mice. This histochemically reactive Zn2+ is enriched in CAA in a transgenic mouse model of AD (Tg2576), and a dramatic reduction of CAA occurs after targeted disruption of the Znt3 gene in these mice. Also, in Znt3 knock-out mice, the amount of exchangeable Zn2+ [detected by N-(6-methoxy-8-quinolyl)-p-carboxybenzoylsulphonamide (TFL-Zn) in the perivascular space was significantly decreased in the neocortex but not in peripheral organs. ZnT3 was not detected in the cerebral vessel walls or in blood components of wild-type mice. Thus, synaptic ZnT3 activity may promote CAA by indirectly raising exchangeable Zn2+ concentrations in the perivascular spaces of the brain.