Fiske and Subbarow1 have isolated phosphocreatine from protein-free muscle filtrates and find “that the compound is destroyed during contraction at a rate which rivals that of glycogenolysis and lactic acid production.” Modern theories of muscle contraction have as their foundation the change of glycogen into lactic acid. But it has been shown that the muscles of insulinized frogs can contract in a normal manner even if no glycogen can be found in them.2To determine whether there is a correlation between glycogen and phosphocreatine we have analyzed individual gastrocnemii of very large frogs for initial phosphorus, phosphocreatine, total phosphorus, lactacidogen, glycogen, and lactic acid.One gastrocnemius was removed with as little stimulation as possible, frozen in liquid air, and weighed portions (0.5 gm. or more) were taken for each of the different estimations. The other gastrocnemius was removed at the same time and stimulated at second intervals nearly to exhaustion, then frozen, portions weighed,...