The significance of glucagon and of the sympatho-adrenal system for insulin secretion and hepatic glycogen depletion during exercise was studied. Male rats were either adrenodemedullated and chemically sympathectomized with 6-hydroxydopamine (SX) or sham-treated (C). During light ether anesthesia, cardiac blood for glucose analysis and a biopsy of the liver were obtained, and either antigen-stripped glucagon antibodies (A) or control gamma globulins (N) in saline were injected through the cardiac cannula. Subsequently, the rats swam in tepid water (33-34 degree C) for 100 minutes with a tail weight attached (2% of body weight). Then cardiac blood was drawn for analysis of glucose, insulin and glucagon, and a sample of the liver was collected. In both CA and CN rats, the blood glucose concentration tended to increase (p less than 0.1) during exercise, whereas hepatic glycogen depletion and the plasma insulin concentration were lower in CA rats compared to CN rats. In SX rats, the blood glucose concentration did not increase during exercise, and in SXA but not in SXN rats, the hepatic glucogen depletion was lower than in CN rats. The plasma insulin concentration was consistently higher in SX rats than in C rats, and was significantly decreased by glucagon antibodies in SX as well as in C rats. In conclusion, in exercising rats, glucagon enhances hepatic glycogen depletion. Furthermore, glucagon and the sympatho-adrenal system increase and decrease, respectively, the plasma insulin concentration.