• Short-term, high-dose steroids (prednisolone sodium succinate) were given to female Lewis rats in the chronic stage of experimental autoimmune myasthenia gravis (EAMG). Abnormally low amplitude miniature endplate potentials (MEPPs) persisted, and were even slightly lower than those seen in saline-treated animals with chronic EAMG. The MEPPs were also studied in normal female Lewis rats treated with short-term, high-dose steroids, and no significant change was noted when compared with normal controls. Previous investigators have reported normalization of the abnormally low amplitude MEPPs within 24 hours of high-dose steroid treatment. Our different results might be explained by our higher dose of receptor, addition of pertussis to the inoculum, and study of a different muscle. A survey of endplate ultrastructure revealed some definite postsynaptic membrane abnormalities in both steroid-treated and saline-treated rats with EAMG, but clear distinction among the groups studied was not possible by direct visualization.