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Cerebrovascular endothelial function can be assessed by measuring the shear-mediated dilation or cerebrovascular reactivity (CVR) of the internal carotid artery (ICA) in response to transient hypercapnia (30 s). ICA CVR is blunted with aging; however, it is not known whether age-related reductions in ICA CVR are influenced by antioxidant-responsive mechanisms. The purpose of this study was to determine whether infusion of the antioxidant, ascorbic acid (AA) restores ICA CVR in middle-aged and older (MA/O) adults. We also examined the effect of AA on middle cerebral artery (MCA) CVR to sustained (3-min) hypercapnia. We hypothesized that MA/O adults would exhibit lower cerebrovascular function compared to young (Y) adults, and that infusion of AA would improve cerebrovascular function in MA/O adults, whereas infusion of saline (SAL) would not. The primary analytic sample included nine Y adults (25 ± 2 years) and nineteen (65 ± 6 years) MA/O adults, with analyzable data varying by outcome. ICA CVR was measured using high-resolution duplex ultrasound, and MCA CVR was measured using transcranial Doppler ultrasound. AA infusion improved ICA CVR and MCA CVR in MA/O adults but had no effect in Y adults. These findings suggest that age-related impairments in cerebrovascular function may be partly attributable to mechanisms responsive to acute, supraphysiological AA administration.