Motor endplate ultrastructure in the biceps brachii was quantitatively analyzed in five patients with ocular myasthenia gravis (56 endplates), six patients with generalized myasthenia gravis (83 endplates), and five controls (64 endplates). Ultrastructural changes of the motor endplates were observed in non weak limb muscles of patients with ocular myasthenia gravis as well as in generalized myasthenia; these changes were mostly restricted to the postsynaptic region. Decrease of the mean presynaptic and postsynaptic membrane length, and postsynaptic membrane density, were more remarkable in generalized myasthenia than in ocular myasthenia. Widening of the primary and secondary synaptic clefts was also observed more frequently in generalized myasthenia. There was no correlation between ultrastructural alterations and the duration of symptoms or treatment, severity of disease, or titers of antiacetylcholine receptor antibody.