Two different jaw opening responses were recorded from the mylohyoid nerve following an electrical or mechanical stimulation of the periodontal ligament in an a-chloralose-anesthetized cat. One featured an early response (ER, 4-6 msec latency) and the other a late response (LR, 16-20 msec latency). The physiological characteristics of the ER were similar to those of the nociceptive response (ER', 4-6 msec latency) evoked by tooth pulp stimulation. The ER had a higher threshold than the LR. Both ER and ER' were more easily depressed than LR by the anesthetization of the inferior dental nerve. These data indicate that ER is produced by the nociceptive receptors of periodontal ligaments, while LR is produced by mechanoreceptors.Both ER and LR were affected by a vibratory stimulation applied to the mandible and by an electrical stimulation of the masseter nerve. ER was suppressed at a frequency range of 50-250 Hz and a vibratory amplitude of more than 250μ; and LR at 40-350 Hz and more than 50μ. These effects were not affected by the anesthetization of the TMJ capsules. Furthermore, ER was suppressed by a stimulation of the masseter nerve with the intensity of a suprathreshold level of the group II fibers, while LR was suppressed by stimulation lower in intensity. These results suggest that ER is controlled by the firing of the secondary spindle afferents of the jaw closing muscles, and that LR is controlled by that of the primary spindle afferents.