Extracorporeal circulation was performed in dogs with a perfusate containing 0.6 to 2.5Gm/dl of hemoglobin and 11 to 20Gm/dl of Perfluorochemical emulsion (PFC). The oxygen consumption increased as the oxygen delivery was enhanced by increasing the perfusion rate. The consumption however, did not increase over a certain value even though the delivery was raised over 8ml/Kg B. W./min. The maximal oxygen consumption was a_little less than 5ml/Kg B. W./min at 37°C, and this amount of oxygen consumpton was obtained when the oxygen delivery increased over 8ml/Kg B. W./min. The minimal perfusion rate necessary to meet the maximal oxygen consumption in dogs was about 120ml/Kg B. W./min when the arterial oxygen content was kept in the range of 6 to 7 volume % by the use of PFC, even though the hemoglobin content was reduced to less than 1.2Gm/dl.