Physiologic and pharmacologic studies provide the present basis for the classification of alpha-adrenergic receptors (Langer, 1974; Berthelsen and Pettinger, 1977; Wikberg, 1978; Starke, 1981). This classification calls for the existence of discrete alpha-1 and alpha-2 adrenergic receptors with the extant possibility of further subdivision among the alpha-2 adrenergic receptors (Cheung et al., 1982; McGrath and Reid, 1985; Bylund, 1985). Until recently, demonstrating the actual physical existence of the unique macromole-cules representing these alpha-adrenergic receptor subtypes has proved elusive. The difficulty in studying their biochemistry is primarily because most membrane-bound hormone receptors, including alpha-adrenergic receptors, are present only at very low concentrations in the cells of target tissues (i.e., frequently less than 0.001% of the total cellular protein). Therefore, a sensitive and reliable means of quantifying receptor activity was required before the biochemical characterization and purification of alpha-adrenergic receptors could progress. The means for the direct measurement of alpha-adrenergic receptor activity was acquired in 1976 with the development of [3H]dihydroergocryptine ([3H]DHE, Williams and Lefkowitz, 1976).