Insulin infused into a sheep fetus over a 3-hr period at the rate of ∼0.24 U-kg-1-h-1increased fetal glucose uptake (utilization) from 4.4 ± 0.7 mg. min-1. kg-1to 6.9 ± 0.9 mg.min-1kg-1as compared to a noninsulin control period. Insulin administration did not alter fetal oxygen consumption (8.6 ± 0.7 ml.min-1.kg-1vs. 7.7 ± 0.7 ml.min-1kg-1), umbilical blood flow (220 ± 1 ml.min-1kg-1vs. 209 ± 16 ml.min-1.kg-1), or the placental clearances of antipyrine (114 ± 7 ml.min-1. kg-1vs. 109 ± 8ml.min-1.kg-1) and urea (24.5 ± 2.2 ml.min-1.kg-1vs. 25.0 ± 2.1 ml.min-1.kg-1). Fetal plasma glucose concentration fell significantly (0.22 ± 0.01 mg.ml-1to 0.16 ± 0.01 mg.ml-1) during insulin infusion. The insulin effect on fetal glucose uptake occurred over a range of maternal glucose concentrations (0.32 → 0.78 mg.ml-1), which were not altered by the infusion of insulin in the fetal compartment.Insulin has a specific effect on increasing fetal glucose uptake and utilization.SpeculationThe effect of insulin on increasing transplacental fetal glucose uptake is consistent with a growth-promoting role for insulin in the fetus. This action of insulin might be due to a direct effect of insulin on the mechanisms of placental glucose transport, to an alteration of placental glucose catabolism, or simply to the effect of insulin on the transplacental gradient of glucose.