Maternal and fetal serum insulin response to glucose, leucine and leucine plus glucose was examined by infusions to normal pregnant women at term immediately before cesarean section. The maternal infusion of glucose (50 g) for 30 or 60 minutes was associated with a marked hyperglycemia and with a rise in serum insulin in mother and fetus. The fetal insulin response to the administration of glucose for 60 minutes was higher (p less than 0.01) than when the same dose was given for 30 minutes, while the blood glucose was lower (p less than 0.01). The maternal infusion of a smaller dose of glucose (25 g) or leucine (15 g) for 60 minutes produced an increase in serum insulin only in the mothers. However the simultaneous administration of these substances stimulated maternal and fetal insulin secretion. The fetal insulin level produced was similar to that seen after the maternal infusion of 50 g of glucose for 60 minutes, while the cord blood glucose was slightly higher but significantly different (p less than 0.01) than that seen in the saline infused group.