THE EFFICACY of hemodilution for cardiopulmonary bypass is well established. However, the degree to which the circulating blood volume can be diluted during whole body perfusion has not been delineated. On the one hand are the proponents of low volume nonblood oxygenator primes1,2 and on the other are the observations by Roe et al3 and Neville et al4 that a much larger system can be used without harmful sequelae. Since both methods entail postoperative infusion of the oxygenator contents to restore hemodynamics, a priori it would seem that the excess fluid in the large prime system could be injurious to the cardiac patient. However, immediate blood volume studies have consistently demonstrated a normal total blood volume with an increased plasma volume and a decreased red cell mass.5 Since normovolemia following this type of perfusion was in sharp contrast to the observations of Gadboys and Litwak7