The facilitation of muscle motor potentials evoked by transcranial magnetic stimulation (TMS) has been demonstrated convincingly during both isometric and dynamic activity in the limbs but not in the jaw muscles. An experimental design involving TMS, surface electromyography and controlled muscle-activity was employed to investigate the motor response of the human masseter during voluntary isometric and dynamic voluntary conditions. During the isometric condition, an increase in muscle facilitation resulted in a progressive increase in motor-evoked potential (MEP) amplitude that was consistently greater on the side contralateral to that subjected to TMS (P < 0.05). No difference in MEP amplitude or laterality of response was revealed for the two dynamic conditions. The sample size may have been too small to reveal any differences. The modulation of MEPs during isometric activity was probably due to cortical and brainstem mechanisms. Putative variation in masseter MEPs during dynamic conditions cannot be discounted.