We studied moderate [fractional inspired O2 (FIO2) 0.09] and severe (FIO2 0.06) hypoxemia in 21 lambs (group 1, 1 wk old; group 2, 3-4 wk old; group 3, 5-7 wk). With moderate hypoxemia, all groups increased heart rate, cardiac output, and pulmonary arterial pressure and decreased systemic and pulmonary vascular resistance; cardiac output and pulmonary arterial pressure increases were less in group 1, because of higher resting values. With severe hypoxemia, heart rate increased similarly, cardiac output was unchanged, and aortic pressure fell associated with severe metabolic acidosis or arrhythmias. Regional blood flow changed similarly in all groups; heart and brain flow increased, carcass flow was unchanged, and skin, GI tract, and kidney flow fell. O2 consumption (VO2) and mixed venous PO2 decreased, though O2 saturation was higher in group 1 due to the lower O2 half-saturation pressure of hemoglobin. With a given decreased VO2 there was more metabolic acidosis in older lambs and an increased VO2 after termination of hypoxemia, suggesting greater O2 debt. As in the fetus, young lambs are better able to tolerate a decreased VO2 than older lambs.