Four patients with the syndrome of continuous muscle fiber activity were seen in a period of 6 years. Young females predominated. Remarkable improvement followed phenytoin sodium and carbamazepine administration in three patients, one of whom was "cured" within 4 years. In the remaining patient the response was inconstant. Electromyography showed abnormal spontaneous activity with diphasic and triphasic potentials appearing as doublets and multiplets. Waxing and waning was observed. D-tubocurarine and succinylcholine abolished the spontaneous activity excluding the muscle and the myoneural junction as its source. Spinal anesthesia, thiopental sodium, sleep and baclofen had no effect on it, ruling out a central source. In three patients, nerve blocks at the knee and elbow or wrist abolished this activity pointing to a proximal site of origin in the nerve somewhere between the spinal cord and the nerve block. In the remaining patient such a block significantly reduced but did not abolished the activity suggesting a dual source above and below the block. Finally successive examinations in one of our patients led us to believe that this activity may arise from different sites at varying times. It appears that regardless of the site of origin of the activity in the motor axon the counter part clinical syndrome remains the same.