The role of the adrenergic system in regulating glycogenolysis during exercise was studied in rats. Alterations in the adrenergic system were produced by injections of 6-hydroxydopamine (6-OHD), surgical removal of the adrenal medulla (ADMX), or the combination of ADMX and 6-OHD injection. Exercise was treadmill running at 22 m/min for 60 min. Colonic temperature averaged 2.8°C higher in the exercised than control rats. Exercise reduced the glycogen of the liver and skeletal muscles of all groups. The glycogen concentrations of the soleus and red portion of the gastrocnemius muscles of the ADMX and ADMX-6- OHD groups were about 3.8 and 2.5 times higher after exercise than those of the normal-exercised rats. Glycogen depletion of the white portion of the gastrocnemius muscle was similar for all exercised groups. 6-OHD treatment depleted the catecholamines of the myocardium. These results demonstrate that glycogen depletion during exercise occurs in rats devoid of adrenergic control. However, differences between types of skeletal muscle suggest that factors other than the adrenergic system may be involved in controlling glycogen metabolism during exercise.