Radioligand binding studies now permit the direct investigation of the alpha- and beta-adrenergic receptors. The concentration of the receptors in cell membranes is modulated by a wide variety of physiological and pathophysiological variables and may be either increased or decreased in different conditions. Ligand binding techniques have also shed light on the ways in which the adrenergic receptors are coupled to other components of the hormone-sensitive adenylate cyclase system. This "coupling" is also a highly regulated step, control of which importantly influences catecholamine and sensitivity of tissues.