The rates of glucose transfer from maternal blood to pregnant uterus and from placenta to fetus were measured in eight sheep at spontaneously occurring glucose concentrations (control state) and while the fetus, the mother, or both were receiving a constant infusion of glucose. In addition two fetuses received insulin infusions. In the control state the net glucose flux from placenta to fetus was only 27 +/- 2.6% (SEM) of the net flux from the uterine circulation to the pregnant uterus. An empirical equation describing the relationship between placental glucose transfer and arterial plasma glucose concentrations was derived from the data and compared with equations constructed on the basis of methematical models of placental function. This analysis indicates that: (1) placental glucose transfer is mediated by carriers with Km approximately equal to 70 mg/dl; (2) the rate of glucose transfer from mother to fetus is limited primarily by the transport characteristics and glucose consumption rate of the placenta; (3) under normal conditions of placental perfusion, glucose transfer is approximately 15% less than it would be if placental blood flows were infinitely large.