While examining a patient with myotonia congenita, it was found that stimulation of the ulnar nerve with frequencies of 5 or 8 cps caused the amplitude of the muscle action potential to decrease rapidly to the base line. During the following lapse, the muscle was paralyzed. After about 20 sec the amplitude increased again, and the strength of the muscle returned. This temporary paresis is apparently of importance in the genesis of myotonic impediment to motility. Probably the disturbance is located in the muscle fibre. Very little information on this subject is available in the literature.