Oxygen availability during cardiopulmonary bypass was assessed in 22 patients under hypothermic and relatively normothermic conditions. The patients were divided into two groups, 17 of whom received ACD blood and 5, CPD blood.The mean P50for all patients fell from a preoperative value of 25.9 ± 2.4 (SD) to 15.6 ± 2.1 during hypothermia confirming a leftward shift of the oxyhemoglobin dissociation curve. Oxygen uptake, calculated from a-&OV0456; oxygen content differences (avDO2) and flow, was significantly lower during hypothermic bypass (65 ± 27 ml/min) than during rewarming (121 ± 41 ml/min). The increase in oxygen affinity during hypothermia was influenced also by changes in acid base and 2,3-DPG concentrations, the changes being similar in both the ACD and CPD groups of patients. During rewarming, however, oxygen availability was increased in the CPD group presumably from significantly increased 2,3-DPG concentrations.A “functional” value of hemoglobin, based upon the effects of the shift of the oxyhemoglobin dissociation curve and, therefore, reflecting the true capacity of hemoglobin to unload oxygen at the tissue level, was calculated. During the hypothermic phase of bypass, this functional hemoglobin was only 4.2 g/100 ml blood, suggesting that, in spite of reduced metabolic demands, oxygenation reserves are minimal.