Rabbit liver glycogen synthase have been purified close to apparent homogeneity in a form dependent on glucose-6-P for full activity. From analyses of the purified enzyme by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, some five polypeptides, apparent molecular weights in the range 79,000 to 90,000, correlated with enzyme activity. The relative abundance of the species varied in different preparations but enzyme could be prepared that was composed almost entirely of a 90,000-dalton polypeptide. Treatment of such enzyme with trypsin generated smaller polypeptides in the sequence 90,000 leads to 85,000 leads to 82,000 leads to 80,000; concomitantly, the enzyme was activated when assayed either in the presence or absence of glucose-6-P. Tryptic proteolysis caused as much as a 16-fold increase in Vmax and a 20-fold increase in the concentration of its substrate, UDP-glucose, necessary for half-maximal activity. The concentration of the activator, glucose-6-P, needed for half-maximal stimulation was decreased 3.5-fold. We propose that rabbit liver glycogen synthase in a glucose-6-P-dependent form has a subunit of apparent molecular weight approximately 90,000, larger than previously reported, and that the enzyme is sensitive to proteolytic degradation.