ABSTRACTHind limb blood flow and O2uptake, mean blood pressure, and heart rate were measured in six fetal sheep at 127‐141 d gestation in the control state and at different levels of hypoxia that were induced by partial occlusion of the maternal terminal aorta. Blood flow was measured by an ultrasonic flow transducer. Control fetal arterial O2content ([O2]a) in the descending aorta was 3.25 + 0.17 mM. In response to graded acute hypoxia, blood flow first increased (22.2versus19.9 mL.min−1.100 g−1,p= 0.003) and then decreased abruptly at approximately 1.5 mM [O2]a.O2uptake decreased about 12% (14.74versus16.71,p= 0.03) as [O2]adecreased to 1.5 mM, and then fell markedly, tending toward zero for [O2]a= 0.9 mM. In the 2.7‐ to 1.5‐mM [O2]arange, heart rate increased above control (194versus169 min−1,p= 0.0024), whereas mean blood pressure did not change significantly. For [O2]a< 1.5 mM, heart rate decreased to 148 min−1(p= 0.0005) and mean blood pressure increased above baseline (55versus47 torr,p= 0.0001). In conjunction with previous evidence, these results define a state of acute moderate hypoxia in which the whole fetus can sustain a relatively high rate of oxidative metabolism, and a state of acute severe hypoxia ([O2]abetween 1.5 and 1.0 mM) in which O2uptake by some fetal organs is selectively and markedly decreased. (Pediatr Res28: 94‐100, 1990)Abbreviation[O2]a, fetal arterial O2content