Maintenance of cerebral perfusion pressure is a prerequisite for the prevention of cerebral ischaemia. Physiological fluctuations in systemic perfusion pressure are compensated by cerebrovascular autoregulation. Cerebral perfusion reserve may be determined by assessing changes in cerebral blood flow using vasodilating stimuli. Transcranial Doppler has been used to assess cerebral blood flow speed in response to those stimuli. We describe a method using Transcranial Doppler with CO2 to analyse cerebral vasoreactivity. The method can be useful to determine hemodynamic reserve in different cerebral ischaemic diseases.