As one of the causes of the space adaptation syndrome, an increased intracranial pressure due to the cephalad fluid shift is suggested. In the present study, we measured intracranial pressure (ICP), aortic pressure and cerebral flow velocity (CFV) in anesthetized rats (n=5) during 4.5 sec of microgravity induced by free drop. The rats were set at horizontal prone (Flat) and 30-degree head-up whole body tilting (HU) positions to examine the effect of gravitational pressure gradient. Then, arterial pressure at the eye level (APeye), cerebral perfusion pressure (CPP; CPP=APeye-ICP), and CPP-CFV relationship was calculated. In HU position, ICP, APeye, and CPP increased by 2.2 +/- 0.4, 12.3 +/- 2.0, and 10.1 +/- 1.7 mmHg respectively. However, CFV did not change significantly. In Flat position, none of these variables did not change significantly. In HU position the slope of CPP-CFV relationship decreased, suggesting the increased cerebral flow resistance. However, it did not change in Flat position. These results can be understood by the disappearance of gravitational pressure gradient by microgravity and the cerebral autoregulation.