The jitter and frequency of blocking was studied at single motor end‐plates in 10 patients with myasthenia gravis (MG) and in a patient with Lambert‐Eaton myasthenic syndrome (LEMS), using single fiber EMG (SFEMG) with axonal microstimulation at rates varying from 0.5 Hz to 20 Hz. While some myasthenic motor end‐plates showed lowest degrees of transmission disturbance at the lowest rates and most pronounced abnormality at the highest rates of stimulation, over one‐half were most abnormal at intermediate rates and improved at higher rates. In 1 patient, all end‐plates behaved in this way. On the other hand, all end‐plates in the LEMS patient showed the expected improvement of the abnormal jitter and blocking on increasing the stimulation rate. It is argued that improvement of jitter and blocking at higher rates, unless dramatic, does not necessarily suggest a presynaptic abnormality.