Cerebrovascular CO₂ reactivity is a vital homeostatic function that helps regulate central pH, and therefore affects the respiratory central chemoreceptor stimulus. With healthy human ageing, both cerebral blood flow (CBF) and ventilatory (VE) responsiveness to hypercapnia are reduced; thus, alterations in CBF and cerebrovascular CO₂ reactivity may affect VE responsiveness to CO₂ with ageing. Although cerebrovascular reactivity to CO₂ is enhanced during exercise, potentially as a means to maintain central CO₂ homeostasis, the extent to which this relationship might be influenced by exercise intensity or ageing is not known. We examined the hypothesis that, both at rest and with progressive exercise, there would be a differential age-related alteration in cerebrovascular and VE responsiveness to CO₂. Ventilation and blood flow velocity in the middle cerebral arterial (MCAv) were monitored continuously in 13 young (mean±SD, 24±5 y) and 11 older (64±5 y) participants at rest and during exercise at 30 and 70% of measured heart rate range (HRR) with and without hypercapnia (5% CO₂; 3 min). Cerebrovascular and VE reactivities to CO₂ were characterised by the slope of their relation to increasing end-tidal PCO₂. During normocapnia, MCAv was lower (all P<0.01) in the older compared to the younger aged group at rest (48.5 v 63.3 cm.s^-1) and during exercise (51.8 v 74.3 cm.s^-1 and 51.0 v 75.9 cm.s^-1, at 30% and 70% HRR respectively). At rest, cerebrovascular reactivity and VE sensitivity were lower in the older compared to younger aged group (2.02 v 2.48 %ΔMCAv.mmHg^-1 and 0.70 v 1.04 L.min^-1.mmHg^-1, respectively), although not significantly (P>0.05). During exercise, however, increases in cerebrovascular reactivity were greater in the older group compared to the responses of the younger group for both the 30% and 70% HRR workloads (4.84 v 3.78 and 5.72 v 3.54 %ΔMCAv.mmHg^-1). In the older group, when compared to the young, VE sensitivity remained lower during exercise (P<0.01) and unchanged (P>0.05) compared to rest. At 70% HRR only, age was directly related to elevations in MCAv-CO₂ reactivity (r=0.64, P<0.01) and inversely related to VE sensitivity (r=-0.54, P=0.01). Collectively, these data indicate that the age-related elevations in CBF-CO₂ reactivity during exercise, especially at higher exercise intensities, may act to attenuate VE sensitivity to CO₂.