1. 1. Phosphorylase activity has been assayed in liver extracts of the frog, Rana esculenta, during the winter period. In native conditions, most of the phosphorylase is present as AMP-independent activity and shows properties similar to those of the a form of the liver enzyme from other vertebrates. 2. 2. It is suggested that regulation of phosphorylase activity is through interconversion between a and b forms operated by endogenous phosphorylase kinase and phosphatase. 3. 3. Kinetic studies show hyperbolic saturation curves for glycogen with apparent K m of 2.91 mM and 9.67 mM for a and b forms, respectively. 4. 4. A hyperbolic saturation curve is also observed for glucose 1-P in the case of phosphorylase a, with an apparent K m of 3.95 mM, whereas a sigmoidal kinetic is shown by the b form for the same substrate; from Hill plots an S 0.5 of 24.2 mM was derived. 5. 5. Hyperbolic responses were observed in the case of AMP, and K a of 70 μM and 0.31 mM were calculated for phosphorylase a and b, respectively.