BACKGROUND: Studies using a transcranial Doppler (TCD) to establish cerebral vasoreactivity (CVR) have mostly focused on the anterior circulation. The purpose of this study is not only to evaluate the feasibility of the power motion mode Doppler (PMD) with a probes fixating device, but also to simultaneously measure the CVR between the middle cerebral artery (MCA) and the basilar artery (BA) during hypercapnea simulated by the rebreathing technique. METHODS: Twenty eight healthy volunteers were enrolled. Baseline hemodynamic values (heart rate, blood pressure) were measured while volunteers sat in a comfortable position for 5 minutes. The TCD was performed in two steps. First, velocities and spectra of the MCAs were simultaneously monitored. Then, the velocities and spectra of the MCA and BA were simultaneously monitored by a headset that included an anterior-posterior probes fixating device. The equation for CVR was ([maximum mean velocity baseline mean velocity] x 100/baseline mean velocity). RESULTS: Baseline mean velocities were revealed as follows: (64.0+/-13.7, 65.0+/-11.9 cm/s in right and left MCA; p>0.05; 67.3+/-12.2, -45.3+/-7.6 cm/s in dominant MCA and BA). CVR did not differ between the dominant MCA and the BA (46.1+/-12.1, 46.0+/-15.1%; p>0.05), nor between the right and left MCAs (46.9+/-15.2, 46.4+/-14.8%; p>0.05). There was a positive linear correlation between the CVR of the dominant MCA and that of the BA (r=0.856; p<0.001). CONCLUSIONS: PMD with a probes fixating device for accurate insonation is a useful tool for evaluating the relative CVR between the MCA and BA. Our study suggests that CVR values of the BA are similar to those of the MCA.