AbstractThe activity of phosphorylase was studied in smooth muscle. The ratio of phosphorylaseatobwas higher in smooth than striated muscle. The total phosphorylase activity in bovine tracheal muscle was approximately 10 per cent of that in rabbit skeletal muscle. Adrenaline, which had a relaxing and glycogenolytic effect in tracheal muscle also stimulated the phosphorylase activity therein. The relaxing and the phosphorylase‐activating effects were not so closely linked, however, as to be suggestive of a causal relationship. In mesenteric arteries where adrenaline increased the tone and depressed glycogenolysis it had no demonstrable effect on the phosphorylase activit