Previous studies showed that the cerebrovasodilation response to hypercapnia is attenuated with aging. The purpose of this study was to determine if normal aging attenuates increases in brain oxygenation during hypercapnia. Prefrontal cortex oxyhemoglobin (OHb) and deoxyhemoglobin (HHb) concentrations were measured in 13 healthy subjects ages 26 to 59 years using a frequency domain tissue oximeter. Measurements were obtained under the following conditions: (1) subject awake breathing spontaneously, (2) during mask ventilation with 21% oxygen, (3) mask ventilation with 100% oxygen, (4) 100% oxygen in a rebreathing circuit to increase end-tidal CO2. Under baseline conditions breathing room air, there was a negative correlation between baseline OHb and age (r=-0.60, P<0.05). Ventilation with 100% oxygen increased OHb without a change in total hemoglobin and no affect of age. During mask rebreathing, end-tidal CO2 increased from 39.5±5.0 mm Hg (millimeters of mercury) to 56.5±5.7 mm Hg, which produced significant increases in OHb and total blood volume that were negatively correlated with age (r=-0.67, P<0.05) and positively correlated to baseline OHb (r=0.60, P<0.05). These results indicate that OHb concentrations decreased with age, consistent with attenuated cerebral vasodilation during hypercapnia.