The effect of electrical stimulation on the activation of phosphorylase in mouse muscle was investigated. Rapid interruption of chemical events was accomplished by simultaneously quick-freezing resting and stimulated paired muscles. Phosphorylase (a/t) ratios (%) were found to be significantly increased after 1 1/2, 3, 15, and 30 sec of stimulation; the increase was not significant after 1/2 sec. The present study shows that phosphorylase is activated with sufficient priority to bring about increased rates of glycogenolysis relatively early during the tetanic contraction of mouse muscle.