The relationships between arterial plasma insulin, glucose, and fructose concentrations during the fed and fasted state were studied in seven fetal lambs and their mothers. A significant correlation between insulin and glucose concentration was noted in all fetal lambs and in their mothers. Fetal sensitivity to glucose, as measured by the slopes of the insulin-response curves, was equal to that of the adult although the fetal response was shifted to the left of the maternal. Glucose infusion in four fetal lambs caused significant insulin elevations but no early insulin response (phase I). Maternal fasting caused no alteration in glucose-induced response in the fetus. Similar glucose infusions in newborn and 1-mo-old lambs demonstrated significant early-phase insulin secretion. Basal insulin to glucose ratios were consistent with an adult pattern as early as 3 days after birth.