Extracorporeal circulation (ECC) without donor blood priming in the pump oxygenator was applied in 23 cases, of which 16 cases needed no blood transfusion (Group Ia) and 7 cases required blood transfusion because of peri-and/or postoperative bleeding (Group Ib). In these two groups, hemodynamic study, blood gas study and chemical determinations were carried out during and after ECC and postoperatively for the initial three days, comparing the results with ten cases (Group Ⅱ) of usual ECC with 20% hemodilution. The results were as follows: 1) The hematocrit value and hemoglobin level of Group Ia and Ib were significantly lower than those of Group Ⅱ during ECC, but recovered to the same level as Group Ⅱ four hours after ECC, and all blood in the pump oxygenator returned. 2) The parameters of hemodynamics, cardiac index (CI), stroke index (SI) and total peripheral vascular resistance (TPR), changed in a similar pattern post-operatively in Group Ia and Ⅱ, whereas in Group Ib, CI and SI were markedly depressed during the first postoperative day but recovered by blood transfusion. 3) Oxygen transport, oxygen consumption and oxygen extraction ratio showed no significant differences among the three groups from four hours through three days postoperatively. 4) No remarkable differences of erythrocyte 2, 3 diphosphoglycerate (2, 3 DPG) and P(50) in vivo were observed during ECC through three days postoperatively in all groups, proving that no hypoxemic state occurred during the period. 5) It was revealed that 2, 3 DPG had statistic linear correlation with P(50) in vivo, oxygen transport, CI and venous oxygen content. These facts were thought to occur as a result of a physiological compensatory mechanism for increasing oxygen delivery. 6) Serum LDH increased markedly at the first postoperative day and remained at a high level even seven days after in all groups. In Group Ia and Group Ⅱ, LDH isozyme 1+2/total increased from the second postoprative day, but LDH isozyme 5/total was slightly elevated as early as during ECC, though at the first postoperative day this fraction recovered to the preoperative level. In Group Ib, LDH isozyme 5/total was observed elevating markedly from four hours after ECC, while LDH isozyme 1+2/total decreased remarkably from the same time, but recovered to the normal range at the first postoperative day. It was thought that peripheral circulatory failure would persist and be improved by blood transfusion. It was concluded that ECC without donor blood prime was secure as the usual ECC with 20% hemodilution from the view points of hemodynamics, blood gas analysis, oxygen delivery and chemical determination. There were many advantages such as saving donor blood and relief of various complications of blood transfusion, when the hematocrit value must be kept above 20%.