The acetylcholine receptor from Torpedo californica electroplax was labeled with the photoaffinity reagent bis(3-azidopyridinium)decane perchlorate. All four receptor subunits (alpha, beta, gamma, and delta) were specifically labeled. In the presence of cholinergic agonists the gamma-, beta-, and delta-subunit labeling was decreased significantly, whereas labeling of the alpha subunit was minimally affected. Full occupancy of the two high-affinity sites involving the alpha subunits in the vicinity of alpha-Cys-192-Cys-193 by covalent reaction with bromoacetylcholine also caused a large decrease of gamma-subunit labeling by the photoaffinity reagent and lesser but significant decreases in beta- and delta-subunit labeling. No decrease in labeling of the alpha subunit was seen. Labeling of the alpha subunit could, however, be inhibited by high concentrations of the agonist carbamoylcholine. We conclude that the binding sites of high-affinity reside at interfaces of the alpha subunit and other subunits and that the alpha subunit also contributes to formation of a low-affinity site(s) for cholinergic compounds.