In a sheep preparation the blood flow to fetal organs was studied 3 to 10 days after surgery by means of the microsphere technique over a range of fetal arterial O2 content from 6 to 1 mM. Blood flows to neural tissues (cerebrum, cerebellum, brain stem), heart, and the adrenals increased in inverse relation to arterial O2 content. As a result the arterial supply of O2 to these organs tended to remain constant over the O2 range studied. Blood flow to the fetal lungs decreased progressively with hypoxia. The blood flow to kidneys, digestive tract, pancreas, and carcass had a tendency to remain constant or increase gradually in the transition from high to moderately low levels of arterial O2 content and then to decrease abruptly in more severe hypoxia. Umbilical blood flow did not change systematically in relation to arterial O2 content.