Cerebral blood flow (CBF) increases from rest to ~60% of peak oxygen uptake (VO2peak) and thereafter decreases towards baseline due to hyperventilation- induced hypocapnia and subsequent cerebral vasoconstriction. It is unknown what happens to CBF in older adults (OA), who experience a decline in CBF at rest coupled with a blunted ventilatory response during VO2peak. In 14 OA (71±10 year) and 21 young controls (YA; 23±4 years), we hypothesized that OA would experience less hyperventilationinduced cerebral vasoconstriction and therefore an attenuated reduction in CBF at VO 2peak. Incremental exercise was performed on a cycle ergometer, whilst bilateral middle cerebral artery blood flow velocity (MCA V mean; transcranial Doppler ultrasound), heart rate (HR; ECG) and end-tidal PCO2 (PETCO2) were monitored continuously. Blood pressure (BP) was monitored intermittently. From rest to 50% of VO2peak, despite greater elevations in BP in OA, the change in MCA Vmean was greater in YA compared to OA (28% vs. 15%, respectively; P<0.0005). In the YA, at intensities >70% of VO 2peak, the hyperventilationinduced declines in both P ETCO2 (14 mmHg (YA) vs. 4 mmHg (OA); P<0.05) and MCA Vmean (-21% (YA) vs. -7% (OA); P<0.0005) were greater in YA compared to OA. Our findings show (1), from rest-tomild intensity exercise (50% VO2peak), elevations in CBF are reduced in OA and (2) age-related declines in hyperventilation during maximal exercise result in less hypocapnic-induced cerebral vasoconstriction. © American Aging Association 2011.