Eleven healthy male volunteers were studied to determine the amount of blood displaced by the inflation of the antishock trousers (AST) at pressures of 40 and 100 mm Hg. Radioisotope scans were used to determine the blood volume distribution and the change in the volume of distribution with inflation of the AST. The volunteers were then phlebotomized approximately one liter of blood and the study was repeated. Less than 5% of the total blood volume was displaced with inflation of the AST. It is unlikely that the clinical improvement seen with inflation of the AST in hypovolemic shock is due to autotransfusion of blood alone.