The discovery of postsynaptic alpha 2-adrenoreceptors with the drug specificities of presynaptic alpha 2-adrenoreceptors has contributed to a refinement of the classification of alpha-adrenoreceptors. postsynaptic alpha 2-adrenoreceptors have been identified by pharmacological means and with the aid of direct radioligand-receptor binding studies. The evidence for the existence of this class of alpha 2-adrenoreceptors in the brain and in vascular smooth muscle is particularly strong. Central postsynaptic alpha 2-adrenoreceptors play a major part in the hypotensive action of centrally acting antihypertensive drugs such as clonidine and alpha-methyl-DOPA. Vascular smooth muscle cells contain postsynaptic alpha 2-adrenoreceptors which mediate vasoconstriction, like the more classical alpha 1-adrenoreceptors. The simultaneous occurrence of contractile alpha 1- and alpha 2-adrenoreceptors in vascular smooth muscle offers a simple model for the characterization of alpha-adrenoreceptor agonists and antagonists. At present, highly selective agonists of alpha 2-adrenoreceptors have been found. These new compounds may be useful for the classification of other alpha-adrenoreceptor populations. It has been suggested that the vascular postsynaptic alpha 2-adrenoreceptor might be located at extrasynaptic sites. Accordingly, adrenaline released by the adrenal medulla would be the endogenous stimulant. Finally, observations have been made in vivo indicating that a transmembrane influx of calcium ions is necessary for linking the drug-induced activation of these alpha 2-adrenoreceptors to vasoconstriction.