The major factor that controls glycogen metabolism in the liver is the concentration of phorphorylase alpha. Indeed, this enzyme catalyzes the limiting step of glycogen breakdown and, by controlling the activity of synthetase phosphatase, also regulates glycogen synthesis. The formation of phosphorylase alpha is stimulated by cAMP, by glycogen, and presumably also by some still ill-defined ionic changes. The ininactivation of phosphorylase is greatly stimulated by glucose and inhibited by AMP and glycogen. Glycogen synthesis is proportional to the concentration of synthetase alpha, which in normally fed animals is formed only when most of the phosphorylase is in the beta form. The inactivation of glycogen synthetase is stimulated by cAMP, an elevated concentration of which puts a double lock on glycogen synthetase by activating phosphorylase alpha (and thereby preventing synthetase activation) and by inactivating glycogen synthetase. The effect of cAMP, 5'-AMP, glucose, and glycogen can presently be explained in molecular terms. The main missing link is in the ionic effect whose elucidation might lead to the understanding of the mode of action of insulin.