Objective. To investigate the change of cerebral blood flow velocity during 4 d head-down tilt (HDT) bed rest in humans. Method. Eight healthy male volunteers were exposed to -6 degrees HDT bed rest for 4 d. Orthostatic tolerance was assessed by means of standard tilt test. Cerebral blood flow velocity in the right middle cerebral artery was measured by transcranial Doppler sonography before HDT (supine), on days 1, 3 and 4 of HDT and on day 2 post HDT. Result. After 4 d HDT, the frequency of orthostatic intolerance was significantly higher than those before HDT (P<0.05). The systolic, diastolic and mean blood velocities in the middle cerebral artery tended to decrease on days 1 and 3 of HDT, and decreased significantly on day 4 of HDT, compared with pre-HDT, and returned to basal levels on day 2 post HDT. The mean arterial blood pressure and diastolic blood pressure increased significantly, while the body weight decreased significantly during HDT. Conclusion. It is suggested that 4 d HDT may decrease cerebral blood flow velocity and increase the frequency of orthostatic intolerance. This decrease of cerebral blood flow velocity may be partially responsible for the increased risk of orthostatic intolerance.