STUDY OBJECTIVE: To test the hypothesis that the reflex and corticobulbar motor excitability of jaw muscles is reduced during sleep. DESIGN: Polysomnographic recordings in the electrophysiological study. SETTING: University sleep research laboratories. PARTICIPANTS AND INTERVENTIONS: The reflex and corticobulbar motor excitability of jaw muscles was determined during the quiet awake state (QW) and quiet sleep (QS) in monkeys (n = 4). MEASUREMENTS AND RESULTS: During QS sleep, compared to QW periods, both tongue stimulation-evoked jaw-opening reflex peak and root mean square amplitudes were significantly decreased with stimulations at 2-3.5 × thresholds (P < 0.001). The jaw-opening reflex latency during sleep was also significantly longer than during QW. Intracortical microstimulation (ICMS) within the cortical masticatory area induced rhythmic jaw movements at a stable threshold (≤ 60 μA) during QW; but during QS, ICMS failed to induce any rhythmic jaw movements at the maximum ICMS intensity used, although sustained jaw-opening movements were evoked at significantly increased threshold (P < 0.001) in one of the monkeys. Similarly, during QW, ICMS within face primary motor cortex induced orofacial twitches at a stable threshold (≤ 35 μA), but the ICMS thresholds were elevated during QS. Soon after the animal awoke, rhythmic jaw movements and orofacial twitches could be evoked at thresholds similar to those before QS. CONCLUSIONS: The results suggest that the excitability of reflex and corticobulbar-evoked activity in the jaw motor system is depressed during QS.