Intravenous or oral administration of concentrated glucose solution into fasted rats simultaneously injected with 14C-bicarbonate resulted in an inhibition of [14C]glucose release into the blood and in an accelerated [14C]glycogen formation associated with glycogen synthetase activation and phosphorylase inactivation in the liver. The specific activity of glycogen was much higher than that of blood glucose after the glucose load, indicating that glycogen originated from gluconeogenesis rather than blood glucose. These metabolic changes induced by the glucose load were not mediated by endogenous insulin because they were observed to the same extent in rats treated with anti-insulin serum. However, they were mostly, if not totally, abolished by adrenalectomy, which suppressed gluconeogenesis and glycogenesis. Glucose tolerance was markedly impaired not only by anti-insulin serum, which inhibits peripheral glucose utilization, but also by adrenalectomy, which affects hepatic metabolism. It is concluded that a glucose load diverts the final product of hepatic gluconeogenesis from blood glucose to liver glycogen; these metabolic changes in the liver are an important determinant of glucose tolerance.