The response to exogenous glucagon in hypoglycemic newborns, control newborns, and infants and children who had been primed with intravenously administered dextrose was evaluated. Dextrose (1 gm/kg) was injected rapidly, and, at 60 minutes, glucagon (0.03 to 0.3 mg/kg) was injected and blood glucose values measured every 15 minutes for one hour. In the infants and children, glucose values following the injection of glucagon rose 43 ± 3 mg/100 ml at 15 minutes and returned to preglucagon values (zero time) by 60 minutes. In both hypoglycemic and control newborns, glucose values following glucagon injection were lower after 15 minutes and continued to fall during the entire 60 minutes. The response to glucagon was significantly less (P<.001) at each time period in the newborn groups when compared to that in the infants and children.