Glycogen synthetase I from rabbit skeletal muscles was studied by electrophoresis in polyacrylamide gel in the presence of sodium dodecyl sulfate. The presence of glycogen in the preparation prevented the destruction of the quaternary structure of the enzyme. In order to separate glycogen synthetase I from glycogen, alpha-amylase from saliva, pig pancrease and bacterial amyloglucosidase were used. The subunit composition of the total preparation and that of the individual glycogen synthetase forms separated ultracentrifugally in the sucrose density gradient, were shown to be identical. The molecular weight of the minimal subunit of glycogen synthetase I from rabbit skeletal muscles was shown to be 36,000. A comparison of the subunit composition of the enzyme preparations stored in the presence and in the absence of phenylmethylsulfanylfluoride did not show that the preparation possesses proteolytic activity.