BACKGROUND: To maintain adequate perfusion, cerebral blood flow (CBF) is preserved by changes in cerebrovascular resistance (CVR) inversely related to fluctuations in mean arterial blood pressure (MAP). It has been hypothesised that during progressive hypotension, a lower limit of cerebral autoregulation (LLCA) is reached beyond which cerebrovascular dilation becomes exhausted and CBF starts to decrease together with BP. We tested this hypothesis by assessing CVR above and below the LLCA. METHODS: Radial arterial pressure, thermodilution cardiac output (CO), and mean middle cerebral artery blood velocity (MCAV RESULTS: For 50 participants undergoing aortic root surgery who met inclusion criteria, LLCA was located at 58 (12) mm Hg, with a corresponding MCAV CONCLUSIONS: The continuing decline of CVR below the LLCA indicated that brain vasculature is still able to react on changing BP. This implies that LLCA should not be regarded as a fixed point but rather a transitional zone between exhausted and normally functioning autoregulation.