The effect of reducing hemoglobin affinity for O2 on fetal oxygenation was assessed in seven fetal lambs in which fetal blood was almost completely replaced by maternal blood 2-3 days postoperatively. Measurements of fetal blood gases and organ blood flow (radionuclide-labeled microsphere technique) were obtained before and 1 h after the exchange transfusion. Umbilical venous blood PO2 increased from 29 +/- 5 to 35 +/- 6 (SD) Torr (P less than 0.001) but hemoglobin O2 saturation decreased from 78.2 +/- 10.3 to 39.8 +/- 8.8% (P less than 0.001), resulting in a 46% decrease in umbilical venous blood O2 content. Since umbilical-placental blood flow also decreased (P less than 0.002), O2 delivery to the fetus decreased by 64% (P less than 0.002). Although O2 extraction increased from 32.5 +/- 6.8 to 50.9 +/- 9.0% (P less than 0.002), fetal O2 consumption fell from 7.28 +/- 1.97 to 4.10 +/- 1.20 ml X min-1 X kg-1 (P less than 0.02), and metabolic acidemia developed. No significant change in fetal cardiac output was observed. Blood flow increased significantly to the myocardium and adrenals but fell in the placenta, carcass, and lungs and was maintained in other organs. This resulted in a significant decrease in the amount of O2 delivered to all fetal organs except to the myocardium in which it was maintained. In the sheep the higher affinity of fetal blood hemoglobin for O2 helps maintain normal oxygenation during fetal life by facilitating O2 uptake at the placenta and unloading O2 in the tissues.