To investigate the effects on perioral muscles with mechanical stimulation to the mucosa electromyograms were recorded in nine subjects. Results were as follows: 1. Tonic excitatory reflex of m. orbicularis oris and m. mentalis was evoked by the non-painful pressure stimulation to lower lip mucosa. These responses were increased, as the stimulus strength was increased. 2. Reflex responses to stimulation of the corner of the mouth and lower lip mucosa were greater than that of buccal mucosa. The response in m. orbicularis oris superior (O.O.S.) to stimulation of upper lip mucosa was greater than in m. orbicularis oris inferior (O.O.I.). And the response in O.O.I. to stimulation of lower lip mucosa was greater than the response in O.O.S. 3. Tonic excitatory reflex was abolished after local anesthesia of the stimulation area. 4. Excitatory reflex was reversed to inhibitory reflex, when background activity of three muscles was changed by voluntary lip closure. When background activity was weak, excitatory reflex was induced by pressure stimulation. On the contrary, when background activity was strong, inhibitory reflex was induced by the same stimulation. When background activity was moderate, excitatory reflex in O.O.I. and inhibitory one in O.O.S. was easily induced by lower lip stimulation. On the other hand, by upper stimulation, excitatory reflex in O.O.S. and inhibitory one in O.O.I. was easily induced. These results suggest that the orthodontic appliance placed on the vestibular oris gives mechanical stimulation to oral mucosa, which induce reflex contraction in perioral muscles.