To examine a hypothesis that change in regional blood flow due to decreased hydrostatic pressure gradient and redistribution of blood during reduced gravity (rG) is different between organs, changes in cerebrocortical blood flow (CBF) and blood flow in the temporal muscle (MBF) with exposure to rG were measured in anesthetized rats in head-up tilt (HUT) and flat (FLAT) positions during parabolic flight. Carotid arterial pressure (CAP), jugular venous pressure (JVP), and abdominal aortic pressure (AoP) were also measured simultaneously. In the HUT group, CBF increased by 15 ± 3 % within three seconds of entry into rG, and rapidly recovered during rG. MBF also increased but the change was significantly greater than that of CBF. JVP increased by 1.8 ± 0.5 mmHg probably due to loss of hydrostatic pressure gradient since measuring point of JVP was 2 - 3cm above the hydrostatic indifferent point. CAP and AoP increased by 16.7 ± 2 and 7.7 ± 2 mmHg, respectively, compared to 1G condition. Muscle vascular resistance {(CAP - JVP)/MBF} decreased on entry into rG, but no significant change was observed in cerebrocortical vascular resistance {(CAP - JVP)/CBF}. In FLAT group, no significant change was observed in all the variables. The results indicate that arteriolar vasodilatation occurs in the temporal muscle, but not in the cerebral cortex. Thus, the blood flow control mechanism at the onset of rG is different between intra- and extracranial organs.