To assess the effect of various degrees of umbilical cord compression on fetal oxygenation, we instrumented fetal lambs at 120 to 128 days' gestation. An electromagnetic flow transducer was placed around the common umbilical artery to record umbilical blood flow flow continuously. Catheters were passed into an umbilical vein and a hind limb artery and vein, and a balloon occluder was placed around the umbilical cord. After recovery from operation, umbilical blood flow was reduced to 75%, 50%, and 25% of control values by controlled cord occlusion. Umbilical venous oxygen content did not change during cord compression; thus, oxygen delivery was linearly related to umbilical blood flow. Oxygen consumption of the fetus was maintained with reduction of umbilical blood flow to about 50% of control values; further reductions were associated with a progressive fall in fetal oxygen consumption. With reduced umbilical flow, there was a progressive increase in oxygen extraction from a control of 33.6% +/- 4.8% to 67.7% +/- 11.3% during 75% reduction of flow.