A [3H]bisazido derivative of ethidium bromide was synthesized to identify sites of interaction of ethidium with the acetylcholine receptor from Torpedo californica and to aid in localization of ligand binding sites. For purified solubilized acetylcholine receptor it was shown (a) that the photolabel was competitive with ethidium bromide, (b) dodecyl sulfate--polyacrylamide gel electrophoresis revealed that all four polypeptide components were labeled with [3H]ethidium azide, and (c) alpha-bungarotoxin inhibited the labeling of the 40 000-dalton subunit. Photolabeling of acetylcholine--receptor enriched membrane fragments led to the following conclusions: (a) the photochemical reaction was more selective than for purified acetylcholine receptor, since the 40 000-dalton subunit was preferentially labeled; this result demonstrated differences in the topography of receptor subunits depending on whether the molecule was in detergent solution or in a membrane-bound state, (b) alpha-bungarotoxin inhibited labeling of the 40 000-dalton subunit, (c) ligand-induced conformational changes resulted in different subunit labeling patterns. The results imply that conformational changes generated at the 40 000 molecular weight subunit upon cholinergic ligand interaction cause further intermolecular structural changes that involve subunits of higher molecular weight. These higher molecular weight subunits therefore belong to a supramolecular complex of polypeptides associated with the postsynaptic membrane.