In order to understand the mechanism of action of the peptide hormone, glucagon, we propose to study the binding of 125I-glucagon to hepatic cells and partially purified liver plasma membranes. Kinetic studies in both systems will be aimed at demonstrating that the biphasic course of glucagon dissociation results from the presence of two interconvertible states of receptor affinity. The proportion of receptor sites in each affinity state will be related directly to cellular sensitivity to further hormonal stimulation. The biologic endpoint employed will be assay of adenylate cyclase activity. Comparison of binding in whole cells and membranes should permit more complete dstinction between processes related to glucagon internalization and degradation and cell surface changes of receptor affinity. The roles of changes of receptor affinity state, and hormone internalization and degradation in modulating hepatic sensitivity to glucagon will be evaluated.