Frog muscles were shown to contain thromboplastin, antiheparin factor and activators of fibrinolysis. The thromboplastin activity was distinctly increased in exhausted muscles. At the same time the activity of fibrinolytic agents was increased less significantly. It was proposed that stimulation of cell structures was related to liberation of thromboplastin from membranes both into an extracellular environment and into the cell. The latter effect caused structural alteration in protoplasm, which resembled blood coagulation. The exhaustion is considered as a result of absence of suitable "spreading out" of protoplasm or as a process caused by decrease in content of structural biopolymers.