We examined the effects of 30 min of exposure to either +3G_X (front-to-back) or +G_Z (head-to-foot) centrifugation on cerebrovascular responses to 80° head-up tilt (HUT) in 14 healthy individuals. Both before and after +3 G_X or +3 G_Z centrifugation, eye-level blood pressure (BP_eye), end tidal PCO₂ (PET_CO2), mean cerebral flow velocity (CFV) in the middle cerebral artery (transcranial Doppler ultrasound), cerebral vascular resistance (CVR), and dynamic cerebral autoregulatory gain (GAIN) were measured with subjects in the supine position and during subsequent 80° HUT for 30 min. Mean BP_eye decreased with HUT in both the G_X (n = 7) and G_Z (n = 7) groups (P < 0.001), with the decrease being greater after centrifugation only in the G_Z group (P < 0.05). PET_CO2 also decreased with HUT in both groups (P < 0.01), but the absolute level of decrease was unaffected by centrifugation. CFV decreased during HUT more significantly after centrifugation than before centrifugation in both groups (P < 0.02). However, these greater decreases were not associated with greater increases in CVR. In the supine position after centrifugation compared with before centrifugation, GAIN increased in both groups (P < 0.05, suggesting an autoregulatory deficit), with the change being correlated to a measure of otolith function (the linear vestibulo-ocular reflex) in the G_X group (r = 0.76, P < 0.05) but not in the G_Z group (r = 0.24, P = 0.60). However, GAIN was subsequently restored to precentrifugation levels during postcentrifugation HUT (i.e., as BP_eye decreased), suggesting that both types of centrifugation resulted in a leftward shift of the cerebral autoregulation curve. We speculate that this leftward shift may have been due to vestibular activation (especially during +G_X) or potentially to an adaptation to reduced cerebral perfusion pressure during +G_Z.