The steady accumulation of glycogen in fetal rat liver during the last fifth of gestation is elicited by a transient rise in the level of circulating corticosterone. One effect of glucocorticoids is to induce glycogen synthase. The actual deposition of glycogen, however, depends on the appearance of a small amount of glycogen synthase in the active, dephosphorylated form. Induction of glycogen synthase phosphatase by glucocorticoids may explain the latter crucial process. Insulin enhances further the rate of glycogen deposition. The effect of insulin requires a previous exposure of the fetal liver to glucocorticoids. It is exerted on the enzyme interconversion system and appears not to involve new protein synthesis. Administration of glucocorticoids to adult fed or fasted animals causes within 3 h an intensive deposition of glycogen in the liver. This phenomenon is ultimately explained by both an activation of glycogen synthase and an inactivation of glycogen phosphorylase. The latter process may be due to an enhanced activity of phosphorylase phosphatase, or possibly of phosphorylase kinase phosphatase. The activation of glycogen synthase is explained by an enhanced activity of glycogen synthase phosphatase. The latter enzyme is normally profoundly inhibited by phosphorylase a; glucocorticoids cause the appearance in the liver of a protein factor that decreases and eventually cancels this inhibitory effect of phosphorylase a. It remains to be established whether or not some part of the glucocorticoid effect on adult liver is mediated by insulin.