Changes in systemic oxygen delivery after acute myocardial infarction were investigated in 21 patients. In seven patients with shock, circulatory failure was characterized by a significant reduction in cardiac index, a decrease in oxygen transport and oxygen consumption and an increase in concentration of blood lactate; a decrease in the affinity of hemoglobin for oxygen (increased P50) was also noted. The P50 averaged 28.8 plus or minus 0.87 (standard error of the mean) torr in patients with shock and 26.0 plus or minus 0.45 torr (P less than 0.05) in patients without circulatory failure. However, there was no significant difference in oxygen extraction from arterial blood between the two groups. The time course of the changes in P50, cardiac index and oxygen consumption was separately examined in 12 patients. In six patients with shock, P50 increased by an average of 4.6 plus or minus 2.05 torr (P less than 0.05) and this augmentation accounted for an estimated 18 percent increase in oxygen release. Maximal P50 values were observed after 24 hours of circulatory failure. In the absence of shock, no consistent changes in P50, cardiac index or oxygen consumption were observed. These data indicate that a reduction in oxygen delivery after acute myocardial infarction is followed by a compensatory increase in P50. This change in P50 accounts for increases in oxygen availability independently of changes in cardiac output.