The problems of extracorporeal circulation remain far from being solved, either from the biological or technical standpoints. However, a major step forward has been the introduction of the principle of hemodilution. This modality has reduced certain risks of perfusion, of which the "homologous blood syndrome"4,5 with its associated changes seem to be the most important. The main technical advantage of the method lies in its simplicity, avoiding to a considerable extent the use of homologous blood for priming the heart-lung machine. However, hemodilution violates the fundamental principle of maintenance of the constancy of the milieu interieur by rapidly altering the concentrations of the constituents of the blood. The changes in the concentrations of electrolytes and the consequent readjustments constitute a major aspect of this problem. The alterations depend on the amount and composition of the solutions used for hemodilution and of the ability of the organism to deal with