Direct radioligand binding methods for studying the alpha- and beta-adrenergic receptors have been developed over the past several years. These techniques use radioactively labeled adrenergic antagonists and agonists to identify the receptors in appropriate membrane fractions from catecholamine-sensitive tissues. In the case of the beta-adrenergic receptors, confident receptor identification has been aided by the close correlation of binding data with data on adenylate cyclase activation. Such direct binding studies are providing new insights about the molecular characteristics and regulatory properties of the receptors.