The reaction of the masseter muscles was studied by giving various loads to the mandible. Five dentulous volunteers and five edentulous patients participated in this study. They were asked to bite two bars in which the upper was immovable and the lower was connected with the torquemotor and to maintain the mandibular position although their mandibles were given the load changes. The load changes corresponded to the food consistency and were performed by the torquemotor rapidly. When the load force increased rapidly, the full-rectified electromyographic activity of the masseter muscle was facilitated 7.5-9.4 ms in the dentulous and 9.3-11.7 ms in the edentulous. Similarly, when the load force decrease rapidly, it was depressed to 11.1-12.9 ms and 10.6-16.4 ms, respectively. The number of synapses in the neuronal mechanism of loading and unloading reflexes showed no difference between the edentulous and the dentulous. Therefore, it is clear that the periodontal mechano-receptor does not cause the loading and unloading reflexes. As a result, the author suggests that the main receptor for the rapid masticatory movement should be the jaw-closer muscle spindle, which implicates that the complete denture wearers can masticate various kinds of foods smoothly.