Peroxidase-conjugated alpha-bungarotoxin (P-BGT) was used for the ultrastructural localization of the acetylcholine receptor in end-plates in external intercostal muscles of four patients with myasthenia gravis, in forelimb digit extensor muscles of rats with advanced chronic experimental autoimmune myasthenia gravis, and in suitable human and rat controls. In control end-plates, the previously reported localization of acetylcholine receptor on the terminal expansions of the postsynaptic folds and, in traces, on the presynaptic membrane was confirmed. By contrast, in myasthenia gravis some postsynaptic regions bound no P-BGT; in other regions, the folds displayed only faint traces of the reaction product, or only some segments of the postsynaptic membrane showed the reaction product; finally, in some regions there was no apparent decrease in reaction product. In general, those postsynaptic regions showing the greatest decrease in P-BGT binding were also the simplest or showed the most degenerative changes, and the presynaptic staining was decreased in proportion to the decrease in the adjacent postsynaptic P-BGT binding. In the experimental animals, the abnormalities in the amount and distribution of the acetylcholine receptor were essentially like those in the more severely affected patients. Morphometric estimates of the postsynaptic acetylcholine receptor surface correlated well with the patients' clinical status and linearly with the miniature end-plate potential amplitude.