The role of cardiac output, blood volume, and peripheral resistance in hemorrhagic shock has been well studied. Changes in the extravascular extracellular fluid and in fluid and electrolytes within the cell, invoked by hemorrhagic shock, have received relatively little attention. The larger portion of the extracellular fluid, comprising 15% of body weight, is the interstitial fluid which lies in an inaccessible area for analysis. This interstitial fluid is surrounded on each side by membranes which have functional integrity. These consist of the capillary membrane on one side and the cell membrane on the other. Consequently, all attempts at measurement of the function of the extracellular fluid in the in vivo state have been confined to indirect methods. It is probably for this reason that the functional extravascular extracellular fluid has received so little attention until the quite recent past. This is remarkable in view of the fact that this huge