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. It is suggested that regulation of phosphorylase activity is through interconversion between a and b forms operated by endogenous phosphorylase kinase and phosphatase. 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. A hyperbolic saturation curve is also observed for glucose I-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 S0.5 of 24.2 mM was derived. 5. Hyperbolic responses were observed in the case of AMP, and K(a) of 70 muM and 0. 31 mM were calculated for phosphorylase a and b, respectively.