Nine samples of purified rabbit skeletal muscle glycogen synthase (UDP-glucose:glycogen 4-alpha-glucosyltransferase, EC 126.96.36.199) were obtained with alkali-labile phosphate contents ranging from 0.27 to 3.49 residues per 85,000 molecular weight subunit. The enzyme samples appeared essentially homogeneous when analyzed by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate and had relatively constant specific activity under standard conditions with saturating UDP-glucose and glucose-l-P concentrations (37.1 +/- 1.0 mumol of glucose incorporated/mg/min). When the UDP-glucose concentration was varied, deviations from Michaelis-Menten kinetics were observed for all samples (Hill slopes of 0.79 +/- 0.02), but these deviations were virtually abolished by the presence of 5 mM glucose-6-P. Glucose-6-P decreased the S0.5 (concentration required for half-maximal rate) for UDP-glucose. The plots of activity increase caused by glucose-6-P versus glucose-6-P concentration became progressively more sigmoid in shape with enzyme samples of higher phosphate content. Both the S0.5 for UDP-glucose and the M0.5 for glucose-6-P (concentration for half-maximal activation) were sensitive functions of the alkali-labile phosphate content of glycogen synthase. The M0.5 increased from 3.3 muM at 0.27 phosphate/subunit to 2.7 mM at 3.5 phosphates/subunit, and the S0.5 varied from 0.75 mM (0.27 phosphate/subunit) to at least 61 mM (2.3 phosphates/subunit). Both parameters increased continuously with phosphate content, with the greatest absolute changes occurring at values greater than 2 residues of phosphates/subunit. The effects of both phosphorylation and activation by glucose-6-P appeared to be mediated primarily through alteration of the apparent affinity for UDP-glucose. With the phosphorylating conditions used, the results suggested that phosphate could not be introduced into the glycogen synthase molecule without altering its kinetic properties. Further, either the different phosphorylation sites were not equivalent, or else identical sites interacted in determining the kinetic properties of glycogen synthase.