To examine acute hemodynamic responses to microgravit (µG) in the head, we measured carotid artery pressure (CAP) and jugular vein pressure (JVP) to calculate cephalic perfusion pressure (CPP=CAP-JVP) and recorded images of microvessels in the iris to evaluate capillary blood flow velocity (CBFV) and capillary diameter (CD) in anesthetized rats during 4.5sec of µG induced by free drop. Rats were placed in 30° head-up tilted (HU, n=7) or horizontal (Flat, n=6) position. In Flat group, none of the measured variables were significantly affected by µG, whereas, in HU group, CAP, JVP, and CPP increased, respectively, by 23.4±2.6, 1.3±0.2, and 22.9±3.1mmHg, and CBFV and CD increased, respectively, by 33±8 and 9±3%, showing an increase in capillary blood flow. To further examine the mechanisms underlying these CAP and JVP increases, another experiment was performed in which CAP and JVP were measured in anesthetized rats (n=6) during a postural change from HU to Flat. In these animals, the change in JVP was similar to that observed during actual µG, but no change in CAP was seen, indicating that the JVP increase during actual µG is caused by disappearance of the gravitational pressure gradient in the head-to-foot axis, whereas the CAP increase is not. In conclusion, actual µG elicits an increase in CPP due to a greater increase in CAP than JVP, resulting in increased capillary blood flow. Although the increase in JVP is explained by the disappearance of gravitational pressure gradient in the head-to-foot axis as a result of µG, the larger increase in CAP is not.