Twenty-four studies were conducted in 14 sheep fetuses to investigate the pancreatic islet a and j3 cell responses to substrate infusions. The studies were conducted between 100 days gestation and near term, using indwelling vascular catheters for infusions and blood sampling. Constant glucose infusions (6 mg/kg/min) produced a significant rise in the mean concentration of insulin in plasma by 105 min, but no change in glucagon in plasma was observed. Near term (>135 days) the mean plasma insulin increment with constant glucose infusion was significantly greater and occurred sooner than in fetuses early in gestation (<110 days), but again no plasma glucagon response was observed. The mean steady state glucose concentration in response to the 6 mg/kg/min glucose infusion was similar early and late in gestation, even though the steady state plasma level of insulin in plasma was higher in the late gestation fetuses. Alanine infused into the fetus did not alter concentrations of glucose, insulin, or glucagon in plasma. These data suggest that changes in plasma concentrations of insulin and glucagon are not important in the regulation of substrate metabolism during fetal life. Moreover, the pancreatic islet β cell secretion response seems to mature earlier than the α cell response. Speculation the failure of glucagon to respond to alanine stimulation during fetal life suggests immaturity of glucagon control of gluconeogenesis from amino acids. The low levels of activity of key glucagon-dependent gluconeogenic enzymes reported in liver of the newborn rat (7, 8) supports this view. These observations suggest the possibility that gluconeogenesis from amino acids may not represent a major metabolic pathway in the sheep fetus.