Exposure to actual and simulated zero gravity causes a significant central or cephalad shift of intravascular and interstitial fluid which triggers a complex set of cardiovascular and systemic adaptations. These adaptations are, in turn, directly responsible for the cardiovascular dysfunction that is apparent after return to normal gravity. However, critical information on several important adaptive mechanisms is incomplete or lacking.An attempt will be made to resolve these problems during a future dedicated life sciences Space Shuttle flight. A series of cardiovascular experiments will utilize direct measurements of central venous pressures, cross-sectional echocardiography, and non-invasive measurements of systemic and peripheral blood flow at rest and during stress. Autonomic control mechanisms will be studied in detail.