Objective There are two theories on the control of the human mandibular rest position; the visco-elastic theory and the muscular theory. In this study, we have searched for evidence for the muscular theory. Design We have investigated the activity of the anterior temporalis muscle during various positions of the mandible using intramuscular electrodes. Results In nine out of ten subjects tonic activity in the anterior temporalis muscle during the mandibular rest position was observed. Most of these units ceased firing under the jaw dropped condition of the mandible. Conclusions Current findings support the muscular theory since single motor unit activity during the rest position of the mandible is observed in the anterior temporalis. We do not however have any evidence against the visco-elastic theory which may contribute in part to maintain the rest position of the mandible.