Recent developments in the characterization of the adrenergic receptors have led to the identification and purification of the binding subunits of the various catecholamine receptors. beta-Adrenergic receptors have been identified in a wide variety of tissues by photoaffinity labeling with the antagonist [125I]p-azidobenzylcrazolol and have been purified to apparent homogeneity from several of these tissues. Thus, beta 1- and beta 2-adrenergic receptor binding sites appear to reside on peptides with molecular weights of 60,000 to 65,000. The alpha 1-adrenergic receptor binding subunit has been identified in several peripheral tissues by photoaffinity labeling with a newly developed probe (4-amino-6,7-dimethoxy-2[4(5(3-[125I]-iodo-4-azidophenyl) pentanoyl)-1-piperazinyl]-quinazoline, or [125I]APDQ). This binding site resides on a peptide with a molecular weight of 80,000. These techniques have been applied to the elucidation of the binding subunit structure of these receptors in the rat central nervous system with the result that beta 1-, beta 2-, and alpha 1-adrenergic binding sites appear to reside on peptides of similar molecular weight to those identified in peripheral tissues (i.e., 60,000-65,000 and 80,000). Using immunocytochemical techniques with antibodies raised to the frog erythrocyte, beta 2-adrenergic receptor, beta-adrenergic receptors were identified at the light microscopic level in regions of the rat and frog brain previously found by ligand binding and autoradiography to be richest in beta-adrenergic receptors. At the electron microscopic level, beta-receptor immunoreactivity was found throughout dendritic processes with local accumulations at certain postsynaptic sites. This finding is consistent with the idea that the density of the receptors might be significantly increased at postsynaptic junctions of adrenergic neurons.