The anatomical and physiological differences between the carotid and vertebrobasilar circulations suggest the possibility of a different response to variations in systemic pO2. We evaluated cerebrovascular response (CR) in these two systems by monitoring variations in the blood flow velocities in the middle cerebral and basilar arteries during hypoxia. Eighteen healthy, non-smoking volunteers underwent transcranial Doppler study during a state of hypoxia obtained by means of the rebreathing method. Oxyhaemoglobin saturation (SaO2) was monitored using a pulsoxymeter in the 88-94% range. The cerebral blood flow velocity (BFV) was measured in the right middle cerebral artery (MCA) and the basilar artery (BA). Our findings indicate that the mean blood flow velocity (MFV) in the BA changes at a lower rate than that in the MCA during hypoxia.