Because hyper-G stress induces peripheral vasoconstriction, hypo-G should lead to vasodilatation. Previous cardiovascular measurements in astronauts in space indicate that cardiac output is increased by some 18% by weightlessness compared to upright standing or sitting on the ground and more so during the initial days of flight than at the end (Prisk et al. 1993). In addition, the mean 24-h diastolic pressure, but not systolic pressure, is significantly decreased in space by 5 mm Hg (Fritsch-Yelle et al. 1996). This is in accordance with observations that very acute weightlessness during parabolic airplane flights and a week of weightlessness in space leads to vasorelaxation, which might be healthy for the circulation (Norsk et al. 2006). That the circulation is dilated in space is in contrast to the augmented sympathetic nervous activity and decreased urine production in space (Norsk et al. 1995; Christensen et al. 1995). These contrasting observations require further investigation, which we plan to do on the International Space Station.