In order to assess the role of glucagon in human protein metabolism and to examine its action as a "catabolic" hormone, studies were conducted in two normal male subjects over an 8-day period. After minimum and stable urinary nitrogen excretion had been produced by the continuous nasogastric administration of carbohydrate (720 g/day) for 8 consecutive days, a continuous intravenous infusion of glucagon (1.0 mg/24 hr) was superimposed on days 7 and 8. Excretion of total nitrogen (N) and urea-N increased significantly (p less than 0.05). Excretion of 3-methylhistidine was unaltered, suggesting that the source of the N losses produced by glucagon did not derive from increased muscle proteolysis. Although striking hypoaminoacidemia was produced, the reductions of extracellular amino acids alone could not account for all of the extra urea excreted. These data suggest that hyperglucagonemia in normal man induces mild nitrogen losses by stimulation of hepatic ureogenesis from free intracellular amino acid pools and not by increased rates of muscle protein breakdown.