Complete conversion of skeletal muscle glycogen synthetase from the I form to the D form requires incorporation of 2 mol of phosphate per enzyme subunit (90,000 g). Incubation of sythetase I with low concentrations of adenosine 3':5'-monophosphate(cAMP)-dependent protein kinase (10 units/ml) and ATP (0.1 to 0.3 mM) plus magnesium acetate (10 mM) results in incorporation within 1/2 hour of 1 mol of phosphate persubunit concomitant with a decrease in the synthetase activity ratio (minus glucose-6-P/plus glucose-6-P) from 0.85 to 0.25. Further incubation for 6 hours does not greatly increase the phosphate content of the synthetase or promote conversion to the D form. This level of phosphorylation is not increased by raising the concentration of protein kinase to 150 units/ml and is not influenced by the presence of glucose-6-P, UDP-glucose, or glycogen. However, at protein kinase concentrations of 10,000 to 30,000 units/ml a second mol of phosphate is incorporated per subunit, and the sythetase activity ratio decreases to 0.05 or less. In addition to the 2 mol of phosphate persubunit which are required for formation of sythetase D, further phosphorylation can be observed which is not associated with changes in synthetase activity. This phosphorylation occurs at a slow rate, is increased by raising the ATP concentration to 2 to 4mM, and is not blocked by the heat-stable protein inhibitor of cAMP-dependent protein kinase. These data indicate that skeletal muscle glycogen synthetase contains multiple phosphorylation sites only two of which are involved in the synthetase I to D conversion.