Immunization of Lewis rats with acetylcholine receptor (AChR) purified from either Electrophorus electricus electric organ or syngeneic rat muscle induced experimental autoimmune myasthenia gravis (EAMG). This was demonstrated by clinical signs of weakness and by electromyographic evidence of imparied neuromuscular transmission. The amount of rat AChR required to induce an autoimmune response was comparable to the amount of eel AChR required. In vitro complexing of rat AChrR with antibody reduced its immunogenicity. Autoantibody to muscle AChR was present in serum and complexed with AChR in muscle. Antibody was not bound to the ACh binding site of AChR, since antibody-AChR complexes extracted from muscle could still bind 125I-alpha-bungarotoxin. The amount of AChR extracted from muscle of rats with EAMG was diminished. The amount of AChR and antibody-AChR complexes in muscle was measured at intervals after immunization with eel AChR. The amount of AChR decreased in rats with acute EAMG, then transiently increased to more than normal amounts during remission, and finally decreased to only about 20% of normal in rats with chronic EAMG. At least half of the AChR remaining in animals with chronic EAMG was complexed with antibody. Thus, both a decrease in amount of AChR and the formation of antibody-AChR complexes contribute to impairment of neuromuscular transmission in rats with EAMG. The possible mechanisms involved in the changes in AChR content are discussed.