Aging is associated with cerebrovascular dysregulation, which may underlie the increased susceptibility to ischemic stroke and vascular cognitive impairment occurring in the elder individuals. Although it has long been known that oxidative stress is responsible for the cerebrovascular dysfunction, the enzymatic system(s) generating the reactive oxygen species (ROS) have not been identified. In this study, we investigated whether the superoxide-producing enzyme NADPH oxidase is involved in alterations of neurovascular regulation induced by aging. Cerebral blood flow (CBF) was recorded by laser-Doppler flowmetry in anesthetized C57BL/6 mice equipped with a cranial window (age=3, 12, and 24 months). In 12-month-old mice, the CBF increases evoked by whisker stimulation or by the endothelium-dependent vasodilators acetylcholine and bradykinin were attenuated by 42, 36, and 53%, respectively (P<0.05). In contrast, responses to the nitric oxide donor S-nitroso-D-penicillamine or adenosine were not attenuated (P>0.05). These cerebrovascular effects were associated with increased production of ROS in neurons and cerebral blood vessels, assessed by hydroethidine microfluorography. The cerebrovascular impairment present in 12-month-old mice was reversed by the ROS scavenger Mn (III) tetrakis (4-benzoic acid) porphyrin chloride or by the NADPH oxidase peptide inhibitor gp91ds-tat, and was not observed in mice lacking the Nox2 subunit of NADPH oxidase. These findings establish Nox2 as a critical source of the neurovascular oxidative stress mediating the deleterious cerebrovascular effects associated with increasing age.