Vascular responses to changes in Paco2 are used widely to estimate cerebral perfusion reserve, and they can also be used to assess the degree of arteriosclerosis. In the present study, the effect of aging on cerebral vascular responses to both hypercapnia and hypocapnia was investigated. Cerebral blood flow was measured with positron emission tomography at rest, during hypercapnia, and during hypocapnia in 11 young men and 12 older men. The vascular response to change in Paco2 was calculated as the percent change in cerebral blood flow per absolute change in Paco2 in response to hypercapnia and hypocapnia. The total vascular response to change in Paco2 from hypocapnia to hypercapnia was also calculated. To evaluate age-related changes in regional cerebral vascular responses on a pixel-by-pixel basis, an anatomic standardization technique was also used. Although no significant differences between young and old subjects was observed for vascular responses to both hypercapnia and hypocapnia, a significant decrease in total vascular response was observed with aging, indicating progression of sclerotic changes in the cerebral perforating and medullary arteries with normal aging. According to anatomic standardization analysis, relative capacities for vasodilatation in the cerebellum and insular cortex, and relative capacity for vasoconstriction in the frontal cortex were greater in the younger subjects. Such aging effects should be considered when estimating cerebral perfusion reserve.