2-Deoxyglucose and 5-thioglucose, in the same fashion as glucose, cause the inactivation of the rat hepatocyte glycogen phosphorylase and the activation of glycogen synthase. However, 6-deoxyglucose and 1,5-anhydroglucitol inactivate phosphorylase without increasing the activation state of glycogen synthase. With 3-O-methylglucose no changes in the activity of these enzymes occurred. These results prove that while glucose is the molecule that triggers the inactivation of phosphorylase, glucose 6-phosphate is the signal for glucose synthase activation and that a metabolite control of the activation state of glycogen synthase is operative in hepatocytes.