Physiological studies hypothesized that unsuppressed gluconeogenesis by insulin in newborn dogs may be a mechanism responsible for neonatal hyperglycemia. In the present study, we determined the effects of fasting and the infusion of insulin, glucose, and/or epinephrine on the liver cytosolic mRNA levels of the gene for the key regulatory enzyme of gluconeogenesis, phosphoenolpyruvate carboxykinase PEPCK (PEPCK; EC 126.96.36.199), in newborn dogs in vivo to further test the hypothesis. We observed the following: (i) Fasting increased the hepatic PEPCK mRNA level in newborn dogs. The hepatic PEPCK mRNA level was not detectable at birth; the PEPCK mRNA level at 4 h was arbitrarily determined as 100.0 +/- 27.8%, was 108.1 +/- 18.4% at 10 h, and stayed at the same level at 24 h (109.1 +/- 8.2). (ii) Euglycemic hyperinsulinemia did not significantly reduce the hepatic PEPCK mRNA levels in newborn dogs; however, the same treatment resulted in the repression of the liver PEPCK mRNA to undetectable levels in adult dogs. (iii) Under hyperinsulinemia, a moderate hyperglycemia lowered the liver PEPCK mRNA in newborn dogs to undetectable levels. (iv) In newborn dogs, despite the presence of hyperinsulinemia and hyperglycemia, the infused epinephrine was still able to elevate the liver PEPCK mRNA from undetectable levels to 79% of the control levels. We suggest that unsuppressed neonatal gluconeogenesis in the presence of hyperinsulinemia may be evidence of insulin resistance in newborn dogs and that the stimulatory effect of epinephrine on gluconeogenesis overriding insulin and glucose in the liver of the newborn dogs may be a mechanism for inducing neonatal hyperglycemia.