The interaction of glucagon and insulin in controlling hepatic glucose production in man has been inferred from studies of immunoreactive glucagon and insulin. This study directly examines the interaction of glucagon and insulin in controlling net splanching glucose production (NSGP) in eight normal men. Glucagon was infused iv at 5 ng/kg/min for 15 min and resultant arterial glucagon levels (1.3 X 10(-10) M) did not exceed the physiologic portal range. In four normal men NSGP increased 2.3-fold by 5 min and remained elevated for 15 min. There was no change in arterial insulin concentration. To study the effect of exogenous insulin on this glucagon-induced increase in NSGP, insulin was infused at 10 mU/kg/min in four normal men to achieve arterial immunoreactive insulin concentrations of 1500 muU/ml (1 X 10(-8) M). Blood glucose was stabilized by glucose infusions. During insulin and glucose administration, NSGP was suppressed and net splanchnic glucose uptake occurred. After 40 min of insulin and glucose pretreatment, a 5 ng/kg/min glucagon infusion resulted in no increase in NSGP (arterial insulin: glucagon molar ratio of approximately 100). In two subjects the glucagon infusion rate was then increased to 15 ng/kg/min (arterial insulin: glucagon molar ratio of approximately 33), resulting in stimulation of NSGP. These studies provide evidence that insulin in high concentration can suppress glucagon-stimulated NSGP in normal man.