In a previous study we found that electrical and chemical stimulation of the red nucleus (RN) suppressed the high-threshold afferent-evoked jaw-opening reflex (JOR). It has been reported that the RN receives bilaterally projection fibers from the raphe magnus nucleus (RMg), and that stimulation of the RMg suppresses the tooth pulp-evoked nociceptive JOR. These facts imply that RMg-induced suppression of the JOR could be mediated via the RN. The present study first examined whether stimulation of the RMg suppresses the high-threshold afferent-evoked JOR. A further study then examined whether electrically induced lesions of the RN or microinjection of muscimol into the RN affects RMg-induced suppression of the JOR. The experiments were performed on anesthetized rats. The test stimulation was applied to the inferior alveolar nerve (IAN) (1 pulse, 0.1 ms in duration, 1 Hz) to evoke the JOR. The stimulus intensity was 4.0 (high threshold) times the threshold. The electromyograms were recorded from the anterior belly of the digastric muscles. The conditioning stimulation (1pulse, 0.2 ms in duration, 1 Hz, 100 μA) was applied to the RMg. The control JOR responses were recorded as well as the modulation induced by stimulation of the RMg. The RN lesion was made bilaterally by the passage of electric current. The effect of the RMg stimulation on the JOR was tested at the termination of lesion. Additionally, localized microinjections of muscimol were made bilaterally into the RN. The control JOR and the effects of RMg stimulation on the JOR at a conditioning-test interval of 40 ms were tested, beginning from the end of the injection. Electrical stimulation of the RMg significantly suppressed the JOR. Significant suppression was observed at a conditioning-test interval of 30-50 ms. Electrically induced lesions of the bilateral RN and microinjection of muscimol into the bilateral RN both reduced the RMg-induced suppression of the JOR. These results suggest that RMg-induced suppression of the high-threshold afferent-evoked JOR is mediated by a relay in the RN.