About 90% of the glycogenin in skeletal muscle extracts prepared from fed, 24-h starved or alloxan-diabetic rabbits sedimented at 140,000 x g with the glycogen/sarcovesicular fraction, from which it was released by glycogenolysis, but not by 1% SDS. Glycogenin in the glycogen/sarcovesicular fraction is therefore bound covalently to glycogen, and not associated (covalently or non-covalently) with the sarcoplasmic reticulum. The same proportion of glycogen synthase was also recovered in the glycogen/sarcovesicle fraction, was solubilised by glycogenolysis, and copurified with glycogenin to yield a heterodimer composed of a 1:1 complex between these proteins. Glycogen synthase and glycogenin are therefore present in equimolar amounts in skeletal muscle and there is an average of one glycogen synthase catalytic subunit associated with each molecule of glycogen in vivo. Glycogenin and glycogen synthase released into the muscle cytosol by degradation of glycogen did not form a complex initially, and only 50% reassociation took place after storage for several hours or overnight dialysis. This suggests that the muscle cytosol may contain a factor(s) which regulates glycogen biogenesis by modulating the association of glycogenin and glycogen synthase. Only glycogen synthase that was complexed to glycogenin was capable of elongating the primer formed by incubation of glycogenin with Mn2+ and micromolar concentrations of UDP-glucose, demonstrating the critical importance of this complex for glycogen biogenesis.