The role of the carotid bifurcation chemoreceptors in mediating the cerebrovascular response to altered arterial Pco2has been suggested to be large. In the present study the cerebrovascular response to raised Pco2was measured in a group of baboons before and after bilateral inactivation of the carotid bodies. The results suggest that these chemoreceptors do play a part in the cerebral vasodilator response to raised Pco2. The role of the carotid body, however, appears to be relatively minor as it only accounted for ± 40% of the total response and became significant only at arterial Pco2levels of more than 50 mm Hg.It is postulated that the peripheral chemoreceptors in the carotid bifurcation mediate part of the cerebrovascular response to altered Paco2but that the role is quantitatively small.