Studies investigating fluid volume dynamics and albumin kinetics in patients undergoing major abdominal surgery indicate that during the operation, marked deficits develop in the total red blood cell volume, plasma volume, extracellular fluid volume, and the total exchangeable albumin pool. A disproportionate deficit in the plasma volume occurring in the course of the operation is believed to be secondary to a functional extracellular fluid volume deficit and abnormal losses of albumin from the intravascular compartment. Deficits in these compartments occurred during the operation in every patient studied. In some patients the plasma volume deficit immediately postoperatively amounted to close to 50% of the volume present when the patient went into the operating room. In the course of a regional node dissection in nine patients studied, a 29% deficit developed in the total body albumin pool.