The regulation of hepatic glucose production by glucagon and insulin has been studied in the intact dog. An attempt has been made to evaluate the role of basal physiological concentrations of the hormones in the regulation of glycogenolysis and gluconeogenesis. Somatostatin was infused continuously into postabsorptive dogs to inhibit the secretion of both glucagon and insulin. Either or both hormones were then replaced intraportally by continuous infusion as desired. The main observations were as follows. (1) When both hormones were simultaneously replaced for periods up to 4.5h, plasma insulin and glucagon concentrations, total glucose output (glycogenolysis plus gluconeogenesis), glucose utilization and the plasma glucose concentration closely matched the same parameters in 0.9% NaCl-infused controls. (2) When glucagon alone was infused, thereby creating a selective insulin deficiency, glucose output (primarily glycogenolysis) rapidly increased by as much as threefold. Glycogenolytic glucose production then fell off progressively and returned to the control value within 4h. The gluconeogenic conversion of [14C]alanine and [14C]lactate into [14C]glucose was stimulated markedly and increased progressively throughout the test period. Glucagon therefore converted the liver from an organ largely dependent on glycogenolysis for glucose production to one heavily dependent on gluconeogenesis. The potent inhibitory effect of basal insulin on postabsorptive glucose output was also clearly apparent. (3) When insulin alone was infused, thereby creating a selective glucagon deficiency, glucose output (glycogenolysis) fell abruptly by about 30% and remained decreased. Gluconeogenesis also decreased (20%) after the selective removal of both insulin and glucagon, but it only remained suppressed for 1h. The low glucose output led to a modest fall in the blood glucose concentration. Thus glucagon plays an important role in maintaining basal glucose production. (4) When insulin was infused and the plasma glucose was kept at its control concentration by infusion of glucose in similar experiments to the above, the hepatic output of glucose fell by as much as 75%. This demonstrates the presence of a glucagon-independent metabolic reflex triggered by a low plasma glucose concentration, the purpose of which is to maintain glucose output at a rate capable of preventing castastrophic hypoglycaemia.