Experiments were carried out on adult cats anaesthetized with Dial (diallyl-barbituric acid) in order to determine whether dental mechanoreceptors are capable of directly influencing jaw muscle activity. Forces were applied to the left maxillary canine tooth and the overall response of the jaw muscles was recorded isometrically with a strain-gauge attached to the mandibular symphysis. Stimulation of the tooth with a force of 400 g applied at 80 kg/sec always elicited a jaw-opening reflex. The response was transitory, lasting about 100 msec independent of stimulus duration. There was no decrease in the response when the gingiva around the tooth was removed, nor when the tooth pulp was extirpated. However, the response disappeared within 5 min following the infiltration of lignocaine over the root of the tooth. Electrical or mechanical stimulation of gingiva, palate or other teeth invariably potentiated the response to tooth stimulation. It is concluded that the mechanoreceptors of the periodontal ligament are capable of reflexly influencing jaw muscle activity.