The effects of intravenous administration of PGE1 on the glycogen synthase and phosphorylase system in rat heart were studied. Unlike the consistent effects of PGE1 on glycogen synthase in liver, the response in heart was variable. A significant decrease in the per cent synthase I occurred in fasted intact rats while a significant increase was seen in adrenalectomized hydrocortisone treated fasted rats. No significant effect was seen on the synthase system in either fed intact or fasted adrenalectomized rats. Phosphorylase a activity was increased significantly following PGE1 administration in fed intact rats and slightly increased in adrenalectomized fasted rats. The phosphorylase system was not affected in fasted intact and fasted adrenalectomized rats given glucocorticoid replacement. With our present state of knowledge an adequate explanation for the response of these heart enzymes to PGE1 under the various conditions of this study does not appear possible.