Electrophysiologic investigations were carried out in 15 patients with limb-girdle muscular dystrophy, two patients with Becker's muscular dystrophy, and 22 patients with dystrophia myotonica. The results were compared with those obtained in 18 patients who had chronic progressive motoneuron disease and in 39 control subjects. These results were within normal limits in muscular dystrophies, with the exception of dystrophia myotonica, in which a neural disturbance is evident in the majority of patients. The results in motoneuron diseases were as expected for neurogenic disorders, loss of motor axons and compensatory increase of the amplitude of muscle potentials. There is no evidence of motoneuron dysfunction in muscular dystrophies, but in dystrophia myotonica, the muscles and nerves are affected independently by the pleiotropic gene of the disease.