The discovery of glycogenin as a self-glucosylating protein that primes glycogen synthesis has significantly increased our understanding of the structure and metabolism of this storage polysaccharide. The amount of glycogenin will influence how much glycogen the cell can store. Therefore, the production of active glycogenin primer in the cell has the potential to be the overall rate-limiting process in glycogen formation, capable of overriding the better understood hormonally controlled mechanisms of protein phosphorylation/dephosphorylation that regulate the activities of glycogen synthase and phosphorylase. There are indications that a similar covalent modification control is also being exerted on glycogenin. Glycogenin has the ability to glucosylate molecules other than itself and to hydrolyze UDPglucose. These are independent of self-glucosylation, so that glycogenin, even when it has completed its priming role and become part of the glycogen molecule, retains its catalytic potential. Another new component of glycogen metabolism has been discovered that may have even greater influence on total glycogen stores than does glycogenin. This is proglycogen, a low molecular mass (approximately 400 kDa) form of glycogen that serves as a stable intermediate on the pathways to and from depot glycogen (macroglycogen, mass 10(7) Da, in muscle). It is suggested that glycogen oscillates, according to glucose supply and energy demand, between the macroglycogen and proglycogen, but not usually the glycogenin, forms. The proportion of proglycogen to macroglycogen varies widely between liver, skeletal muscle, and heart, from 3 to 15% to 50% by weight, respectively. On a molar basis, proglycogen is greatly in excess over macroglycogen in muscle and heart, meaning that if the proglycogen in these tissues could be converted into macroglycogen, they could store much more total glycogen. Discovering the factors that regulate the balance between glycogenin, proglycogen, and macroglycogen may have important implications for the understanding and management of noninsulin-dependent diabetes and for exercise physiology.