ABSTRACT. The effects of euglycemic hyperinsulinemia on hepatic and muscle metabolism were determined in the fasted newborn dog during the first day of life. Hyperinsulinemia was sustained with a primed constant infusion of insulin whereas euglycemia was maintained with an intravenous infusion of 10% glucose using the insulin clamp technique. Euglycemic hyperinsulinemia caused an increase of glucose utilization from 43.9 f 3.7 to 66.5 + 5.4 ~mol/kg/min ( p < 0.001) and reduced endogenous glucose production to 44.4 f 5.4% of basal values obtained before the induction of hyperinsulinemia. Hepatic tissue glycogen, triglycerides, or intermediates were not altered by hyperinsulinemia nor was the incorporation of [3Hjglucose into glycogen. However, the hepatic cytoplasmic redox state was more oxidized, and the incorporation of [3Hjglucose into triglycerides was higher among hyperinsulinemic pups. Pups who demonstrated incomplete suppression of endogenous glucose production had metabolite perturbation suggestive of ongoing gluconeogenesis. Despite very few changes in hepatic tissue metabolite levels, pups subjected to hyperinsulinemia demonstrated a linear uptake of 2-