Observations from several laboratories in the past few years have led to the current belief that phosphorylase catalyzes glycogenolysis but not glycogenesis in the muscle cell (l-7). In resting muscle, phosphorylase has been found predominantly (60 to 70 %) in the inactive or b form (8, 9). When glycogenolysis occurs, for example, after stimulation by epinephrine (9, 10) or electrical shock (11)) the amount of active phosphoiylase, or the a form, increases at the expense of the b form. Under these conditions total phosphorylase (a plus b) does not change (9-11). A curious observation made in this laboratory (12) could not be explained by this current concept. In resting skeletal muscle of one inbred strain of mice, the I, the amount of phosphorylase a was almost negligible, 0 to 57,, but .in another strain of mice, the Cs7, the amount of phosphorylase a was 60% of the total; the total enzyme was similar in both strains (12). Although glycogenolysis is known to occur in both strains of mice, either in vitro’ or in viva (13), the amount of phosphorylase a in the skeletal muscle of the I strain has never under any conditions increased to more than 9% of the total, whereas, as shown in this report, the amount of phosphorylase a in skeletal muscle of C5, strain mice increases to as much as 90% of the total enzyme. The levels of muscle glycogen are 3to 4-fold greater in the I than in the C5, strain animals, and, superficially at least, the differences in proportion of phosphorylase a would seem to provide a reasonable explanation for this observation (12, 13). However, the means whereby glycogenolysis is achieved in I strain animals is not clear, unless one assumes that low levels of phosphorylase a are sufficient. The experiments reported here were initiated in the hope of finding conditions by which significant amounts of phosphorylase a could be demonstrated in I strain mice. No such conditions have been found in skeletal muscle. Glycogenolysis occurred in both strains of mice during exercise or after epinephrine stimulation. The amount of phosphorylase a increased in the musculature of Cr,r strain mice given epinephrine, but little or no change was found in I strain mice. In the heart, an increase in phosphorylase a could be induced in both strains of mice, with a concomitant glycogen breakdown.