Glycogen synthase and glycogen phosphorylase activities were determined in skeletal muscle from lean and genetically diabetic (db/db) mice. The activities of both forms of glycogen synthase and of glycogen phosphorylase in diabetic mice were found to be comparable to those activities observed in muscle tissue from lean animals. Enzyme activities in muscle from diabetic animals remained essentially unaltered throughout the 12-week period studied while glycogen content was consistently depressed, despite the presence of hyperglycemia, hyperglucagonemia and transient hyperinsulinemia. These findings suggest that an impairment in muscle glycogen synthase activation may exist in db/db mice in vivo and that skeletal muscle glycogen synthesis may be resistant to the action of insulin.