An investigator in the field of endocrinology is ever faced with the fascinating and challenging problem of locating the point of action of a given agent. Sutherland and Cori (17) were the first to determine that the principal site of action of epinephrine in its glycogenolytic role was at the level of phosphorylase. Later Rall et al. (16) and Sutherland and Rall (18) determined that this effect was mediated by cyclic 3',5'-AMP. The present contribution represents a continuation of this same general theme. We would like to know, in as specific terms as possible, what component of the phosphorylase system actually combines with cyclic 3',5'-AMP and is responsible for its function. As has been pointed out (9, 10) the site of action appeared not to be on phosphorylase itself but rather on the enzyme, phosphorylase b kinase, which activates phosphorylase. Now, as a result of further investigation, the evidence is accumulating that the nucleotide may act even earlier in this complex array of regulatory components and reactions. Further work on the mechanism of phosphorylase b kinase activation will be required before the site of action of cyclic 3',5'-AMP can be pinpointed with greater accuracy.