Umbilical glucose and lactate extractions were determined in previously instrumented pregnant ewes into some of which D-glucose was infused to produce graded levels of maternal hyperglycemia as great as 20 mM. While fetal arterial glucose concentration continued to increase linearly as a function of maternal arterial glucose concentration during maternal hyperglycemia, the umbilical venoarterial difference in blood glucose concentration did not, and umbilical glucose extraction approached a plateau at approximately 0.063 mmol X min-1 X kg fetus-1 at maternal glucose concentrations greater than approximately 8 mM. The observed plateau in glucose extraction is consistent with saturation at high maternal glucose concentrations of the carrier mechanism for transport of glucose from the maternal to the fetal aspects of the trophoblast. The observed value of the plateau in umbilical extraction of glucose is slightly less than the maximum extraction predicted from previously published equations for this species, but the maternal blood glucose concentration at which the observed maximum occurred agrees closely with the value predicted by those equations. Umbilical lactate extraction, 0.031 +/- 0.021 mmol X min-1 X kg fetus-1, was independent of maternal arterial blood glucose and lactate concentrations and was independent of umbilical glucose extraction.