Objective: The purpose of this study was to evaluate the effects of aging on cerebral artery structure and function. Specifically, we examined changes in smooth muscle (myogenic) vs. endothelial (nitric oxide) influences on tone and reactivity to pressure, along with dimensional and biomechanical properties of small cerebral arteries isolated from young (4-6 weeks), adult (14-16 weeks) and older (44-47 weeks) rats. Methods: Secondary branches of the posterior cerebral artery (PCA, 100-200 mm diameter) were isolated, cannulated and pressurized, and then subjected to a wide range of intraluminal pressures to determine myogenic tone and reactivity with and without pharmacologic NOS inhibition. Measurements of lumen diameter and wall thickness were used to calculate wall : lumen ratios, cross-sectional area, wall stress and distensibility. Results: Although pressure-dependent tone and reactivity decreased significantly during aging, differences between treatment groups were abolished by eNOS inhibition. Aging was also associated with reductions in lumen diameter, along with increased wall thickness (and wall:lumen ratio) and cross-sectional area. This pattern of inward hypertrophic remodeling reduces wall stress in old animals and was not associated with any significant changes in distensibility. Conclusion: In summary, aging is associated with changes in cerebral artery structure, reactivity, and endothelial function in a way that predicts a decrease in cerebral blood flow and vasodilator reserve, and compromised blood flow autoregulation.