Background: The purpose of this study was to investigate cerebral blood fl ow (CBF) velocity in humans during 21 days of head-down tilt (HDT) bed rest with and without lower-body negative pressure (LBNP). Material/Methods: Twelve healthy male volunteers were exposed to –6° HDT bed rest for 21 days. Six subjects received –30 mmHg LBNP sessions for 1 h per day from the 1[sup]st[/sup] to the 7[sup]th[/sup] day and from the 15[sup]th[/sup] to the 21st day of HDT, and six others served as controls. CBF velocity was measured by use of the transcranial Doppler technique in the right middle cerebral artery before and during HDT. Results: In the control group, mean and systolic CBF velocities decreased on day 1 of HDT compared with the pre-HDT value, and dropped further on day 3 of HDT, then remained signifi cantly below the pre-HDT baseline on days 7 and 10 of HDT, and reached a minimum value on day 21 of HDT. In the LBNP group, mean and systolic CBF velocities decreased signifi cantly on day 1 of HDT compared with the pre-HDT value, and remained lowered throughout HDT. Diastolic CBF showed no signifi cant change throughout HDT in both groups. There were no signifi cant differences in these parameters between the two groups. Conclusions: The results of this study suggest that CBF velocity is reduced during 21 days of HDT, and brief daily LBNP sessions used in the fi rst and last weeks of 21-day HDT bed rest does not improve CBF velocity.

Keywords: Head-Down Tilt - adverse effects, Bed Rest, Blood Pressure, Cerebrovascular Circulation, Head-Down Tilt - adverse effects, Lower Body Negative Pressure, Ultrasonography, Doppler, Transcranial, Weightlessness