Two days after catheter placement we measured the heart rate, arterial blood pressure, myocardial blood flow, and the myocardial consumption of oxygen, glucose, lactate, and pyruvate in 11 fetal sheep in utero. We then administered 8-10% oxygen to the ewe, producing a 50% decrease in oxygen content in the fetal ascending aortic blood. After 15 min of hypoxemia we repeated the measurements. Oxygen content in the fetal coronary sinus blood decreased significantly, but the arteriovenous difference of oxygen across the left ventricle also decreased during hypoxemia. Fetal myocardial blood flow increased 160% above the control level, and the myocardial oxygen consumption did not change. The systolic arterial blood pressure increased and the heart rate decreased, but cardiac work, as estimated by the rate-pressure product, was unchanged. As both fetal myocardial oxygen consumption and cardiac work did not change, myocardial oxygenation, the relationship between oxygen consumption and cardiac work, appears to be unchanged during this degree of hypoxemia. Although arterial blood glucose, lactate, and pyruvate concentrations increased significantly during hypoxemia, only the myocardial consumption of pyruvate increased; the arteriovenous difference of glucose and lactate decreased in proportion to the increase in myocardial blood flow. During hypoxemia, glucose consumption did not change, and lactate continued to be consumed rather than produced; thus it is apparent that fetal myocardial metabolism continued to be aerobic during this degree of hypoxemia. Complete oxidative combustion of the quantities of carbohydrates that were consumed would supply all of the substrate necessary to meet fetal myocardial energy demands both at rest and during hypoxemia.