Using the transcranial Doppler technique, the carbondioxide reactivity of the blood flow velocity in the human basilar artery (BA) was investigated in healthy volunteers for comparison with that in the middle cerebral artery (MCA). An exponential curve with an exponent of 0.044 mmHg-1 was found to be a good fit to the BA data and 0.040 mmHg-1 to the MCA data, respectively. While the response of both arteries showed a good correlation to the end-tidal carbondioxide partial pressure (PETCO2), there was no significant difference observed between the carbondioxide (CO2) reactivity of each artery. In both arteries, the end-diastolic flow velocity decreased significantly with the reduction of PETCO2, which may represent the increasing resistance of cerebral microvasculature in hypocapnia. Because of the reliability and noninvasiveness of this method, this normal value should be quite helpful for the bed-side evaluation of an altered CO2 response in the patients with cerebrovascular disease.