The rate of O2 delivery to the pregnant uterus (FaO2) was decreased in chronic sheep preparations by mechanical occlusion of uterine blood flow. The relationship of uterine venous O2 saturation (SVO2) to FaO2 was curvilinear with convexity toward the SVO2 axis. As SVO2 decreased, there was a decrease in uterine and umbilical venous O2 tension (PO2), with no appreciable reduction of the PO2 difference between the two veins and a decrease in the umbilical vein O2 delivery rate. Fetal O2 uptake and base excess remained normal as the umbilical vein O2 delivery rate was reduced from 1.1 to 0.6 mmol . min-1 . kg-1 but decreased markedly at an O2 delivery rate less than 0.5. Umbilical venous CO2 tension (PCO2) was higher than, and strongly correlated with, uterine venous PCO2 (R = 0.954). These observations support a venous equilibration model of ovine placental exchange and demonstrate that under normal physiological conditions the O2 supply to the fetal lamb is approximately twice the value necessary to maintain an adequate fetal O2 uptake and a normal fetal base excess.