Lower esophageal sphincter function and gastric acid secretion were studied in a patient with endogenous hyperglucagonemia due to a functioning islet cell carcinoma. Complete resection of the tumor resulted in a fall of the serum concentration of immunoreactive glucagon to a normal level. Pre- and postoperative resting lower esophageal sphincter pressures and lower esophageal sphincter pressure responses to administration of pentagastrin, edrophonium, and bethanechol were unchanged. After surgery, preoperative immunoreactive glucagon concentrations were reproduced by intravenous infusion or intramuscular injection of exogenous glucagon. Lower esophageal sphincter resting pressures and responses to agonists were unchanged. In contrast, glucagon administered at 36 micrograms/kg/hr, which produced a serum concentration of immunoreactive glucagon (32,000 pg/ml) much greater than observed preoperatively (1200 pg/ml), diminished resting lower esophageal sphincter pressure and sphincter responses to pentagastrin, edrophonium, and bethanechol. Similarly, pentagastrin-stimulated gastric acid secretion was unaffected by tumor resection or low-dose glucagon infusion but was decreased at a glucagon infusion rate of 36 micrograms/kg/hr. This series of observations supports the thesis that endogenous glucagon plays no physiological role in the regulation of lower esophageal sphincter pressure or gastric acid secretion.