Masticatory muscle silent periods were compared in normal subjects and in patients with MPD syndrome before and after a variety of psychological, physiological, and pharmacological forms of therapy. All patients initially showed prolonged silent periods in one or more of the muscles. There was a direct relationship between prolongation and severity of symptoms. The temporalis (90.9%) and masseter (54.5%) were most frequently involved. In almost all instances the silent periods returned to the normal range following treatment.