Crude fresh membranes from rat liver and membranes from rat heart obtained according to Snyder and Drummond were tested for adenylate cyclase activation by glucagon (Gn) and seven glucagon analogs including (Ala2)-, (Arg12)-, (Des-His1, Arg12), (Phe1, Arg12)-, (N-Ac-His1, Arg12)-, (1-Me-His1, Arg12)-, and (3-Me-His1, Arg12)-glucagon. (Des-His1, Arg12)-glucagon acted as a competitive antagonist in heart membranes and as a partial agonist in liver membranes. Results obtained with analogs where His1 was modified suggest that the size of the imidazole ring and the charge of its nitrogen 1, but not the charge of the free amino group of histidine, played a major role in biological activity. When comparing functional glucagon receptors in liver and heart membranes, it appears that the first receptors were more sensitive to the hormone and more efficiently coupled to adenylate cyclase.