It is well known that stimulation of the heart by epinephrine and other sympathomimetic amines causes activation of glycogen phosphorylase, ie conversion of phosphorylase b to phosphorylase a, either in the intact animal or in the isolated organ (l-5). This reaction appears to be analogous to that occurring in skeletal muscle (6-8) and is catalyzed by phosphorylase b kinase. In skeletal muscle, the kinase, like phosphorylase itself, exists in two forms which have been interpreted as being nonactivated and activated forms of the enzyme (9-11). Nonactivated phosphorylase b kinase is almost inactive at pH 6.8 and is only partially active at higher pH values as compared to the activated form of the enzyme. An increase in the ratio of activity at pH 6.8 to activity at pH 8.2 has been used as a measure of kinase activation (12, 13). Although phosphorylase b kinase has been detected in cardiac tissue (8), the heart enzyme has not been studied extensively despite a high degree of interest in the interconversion reactions of phosphorylase in this organ. One of the aims of the present study was to carry out a preliminary characterization of heart phosphorylase b kinase particularly in reference to its response to activating factors which affect the skeletal muscle enzyme (9-11, 14). Another aim was to obtain direct evidence in the intact organ for what has generally come to be accepted (15) as the mechanism for phosphorylase activation in the heart by catecholamines; namely that (a) epinephrine causes the formation of cyclic AMP (16);(b) increased levels of cyclic AMP promote the activation of phosphorylase b kinase (9); and (c) activated phosphorylase b kinase converts phosphorylase b to phosphorylase a. Most of the work on which this hypothesis is based has been done ile vitro with extracts or partially purified enzyme preparations, and in some instances the concepts have been derived from experiments done on tissues other than the heart. With the availability of a method for measuring cyclic AMP levels in tissues (17), it became desirable to test this hypothesis by simultaneous measurements of cyclic AMP levels, phosphorylase b kinase activity, and phosphorylase activity in isolated, perfused hearts stimulated with epinephrine.