Upright standing is associated with a substantial decrease in plasma volume. This postural decrease in plasma volume is due to a shift in fluid from the intravascular space to the interstitium. The magnitude of this fluid shift may be related to the amount of subcutaneous tissue which has a relatively low interstitial pressure. The profound increase is subcutaneous tissue observed in obesity may lead to an increased loss of plasma volume upon standing. An increased postural plasma volume loss in obesity may be compensated for by release of catecholamines and activation of the renin-angiotensin-aldosterone system. Dynamic postural plasma volume loss may also concentrate blood consituents which could increase coaguability of blood. These responses may contribute to an acutely incfreased cardiovascular risk and chronic changes of cardiovascular structure in obese subjects. It is also possible that agents which alter tone in the vasculature may increase or decrease plasma colume loss on standing.