We investigated whether dynamic cerebral autoregulation is affected by exhaustive exercise using transfer-function gain and phase shift between oscillations in mean arterial pressure (MAP) and middle cerebral artery (MCA) mean blood flow velocity (V(mean)). Seven subjects were instrumented with a brachial artery catheter for measurement of MAP and determination of arterial Pco(2) (Pa(CO(2))) while jugular venous oxygen saturation (Sv(O(2))) was determined to assess changes in whole brain blood flow. After a 10-min resting period, the subjects performed dynamic leg-cycle ergometry at 168 +/- 5 W (mean +/- SE) that was continued to exhaustion with a group average time of 26.8 +/- 5.8 min. Despite no significant change in MAP during exercise, MCA V(mean) decreased from 70.2 +/- 3.6 to 57.4 +/- 5.4 cm/s, Sv(O(2)) decreased from 68 +/- 1 to 58 +/- 2% at exhaustion, and both correlated to Pa(CO(2)) (5.5 +/- 0.2 to 3.9 +/- 0.2 kPa; r = 0.47; P = 0.04 and r = 0.74; P < 0.001, respectively). An effect on brain metabolism was indicated by a decrease in the cerebral metabolic ratio of O(2) to [glucose + one-half lactate] from 5.6 to 3.8 (P < 0.05). At the same time, the normalized low-frequency gain between MAP and MCA V(mean) was increased (P < 0.05), whereas the phase shift tended to decrease. These findings suggest that dynamic cerebral autoregulation was impaired by exhaustive exercise despite a hyperventilation-induced reduction in Pa(CO(2)).