Glucagon excess causes catabolic changes, including enhanced glucose production, lipolysis, and amino acid oxidation. In this study, we evaluate the metabolic effects of debulking surgery on a patient with glucagon-producing tumor. Stable isotope tracer methods were used to measure glucose, glycerol, and α-ketoisocaproic acid (αKICA) rates of appearance (Ra) into plasma. Measurements were obtained 25 days after surgery in the basal state and during hormonal suppression of glucagon production by infusing somatostatin with insulin replacement. Basal plasma glucagon concentration (14, 100 pg/mL) remained high after debulking surgery. Somatostatin infusion decreased plasma glucagon concentration to 6, 735 pg/mL and basal substrate kinetics (α-KICA Ra from 1.97 to 1.48 μmol/kg/min; glucose Ra from 16.89 to 11.56 μmol/kg/min; and glycerol Ra from 3.33 to 2.74 μmol/kg/min). We conclude that debulking surgery fails to adequately suppress glucagon production and the alterations in substrate metabolism associated with excess glucagon. In these patients, somatostatin therapy can be an effective method to suppress secretion of glucagon and help attenuate its catabolic effects.