Glycogen synthase from rabbit muscle was examined with the electron microscope. In preparations of the completely converted glucose-6-phosphate dependent form (GSD) and the independent form (GSI) three structures were observed: toroids, hexagons and stacks of four elements which appear to be aggregates of four toroids. Toroids can be formed from hexagons by radial inward movement of subunits which form the vertices of the hexagons. Analysis of the dimensions of these structures and comparison of the known chemistry of the enzyme to the subunits as inferred from electron microscopy suggests a model for the structure of glycogen synthase. The model allows predictions of types of subunits in the enzyme, their relation to phosphorylatable and -SH sites and the possibilities of control by small effector molecules.