T o evaluate the effects of physiologic hyperglucagonemia on splanchnic glucose output, glucagon was infused in a dose of 3 ng/kg per min to healthy subjects in the basal state and after splanchnic glucose output had been inhibited by an infusion of glucose (2 mg/kg per min). In the basal state, infusion of glucagon causing a 309 +/- 25 pg/ml rise in plasma concentration was accompanied by a rapid increase in splanchnic glucose output to values two to three times basal by 7-15 min. The rise in arterial blood glucose (0.5-1.5 mM) correlated directly with the increment in splanchnic glucose output. Despite continued glucagon infusion, and in the face of stable insulin levels, splanchnic glucose output declined after 22 min, returning to basal levels by 30-45 min. In the subjects initially receiving the glucose infusion, arterial insulin concentration rose by 5-12 muU/ml, while splanchnic glucose output fell by 85-100%. Infusion of glucagon causing an increment in plasma glucagon concentration of 272 +/- 30 pg/ml reversed the inhibition in splanchnic glucose production within 5 min. Splanchnic glucose output reached a peak increment 60% above basal levels at 10 min, and subsequently declined to levels 20-25% below basal at 30-45 min. These findings provide direct evidence that physiologic increments in plasma glucagon stimulate splanchnic glucose output in the basal state and reverse insulin-mediated inhibition of splanchnic glucose production in normal man. The transient nature of the stimulatory effect of glucagon on splanchnic glucose output suggests the rapid development of inhibition or reversal of glucagon action. This inhibition does not appear to depend on increased insulin secretio.