alpha-Adrenergic receptors have been grouped into two major subtypes, termed alpha 1- and alpha 2-receptors. Radioligand binding techniques have been utilized to measure the number of alpha 1- and alpha 2-receptors in a variety of tissues. [3H]Dihydroergocryptine labels the entire alpha-receptor population; the alpha-receptor subtypes may be delineated by constricting competition curves with unlabeled selective antagonists and analyzing the data with computer modeling techniques. Alternatively, alpha 1- and alpha 2-receptors may be directly identified with selective radioligands such as [3H]prazosin and [3H]yohimbine, respectively. For example, rat liver membranes have been shown to contain alpha 1- (80%) and alpha 2- (20%) receptors; the alpha 1-receptors activate glycogen phosphorylase. Radioligands have also been used to probe the mechanism by which alpha 2-receptors may inhibit adenylate cyclase activity. Agonist competition curves with [3H]dihydroergocryptine at eht human platelet's alpha 2-receptor may be resolved into two affinity components, interconvertible by guanine nucleotides. These data suggest the agonist-promoted association of the alpha 2-receptor with an additional membrane component. More direct evidence in favor of this possibility was indicated by the increase in sedimentation velocity of solubilized agonist-labeled receptor on sucrose density gradients.