Aims: Since sleep bruxism (SB) is characterized by grinding and clenching of the teeth during sleep and could be an exaggerated manifestation of normal spontaneous rhythmic masticatory muscle activity, the aim of this study was to obtain a neurophysiological assessment of the excitability of the central jaw motor pathways in patients with signs and symptoms suggestive of SB. Methods: A total of 30 subjects diagnosed with SB on the basis of self-report of tooth grinding were studied using the "recovery cycle" of the masseter inhibitory reflex (MIR) elicited by electric and magnetic stimulation of the mental nerves and by recording the motor potentials evoked in masseter muscles by transcranial magnetic stimulation. Tests were done during daytime, when the subjects were awake. The data obtained were compared with data from a population of normal subjects. Results: In the putative SB patients and in normal subjects, the MIRs evoked by single electric and magnetic stimuli were similar. With paired stimuli, the degree of suppression of the late silent period was significantly lower (P .01) in the patients compared to normal subjects, particularly for magnetic stimuli, at various interstimulus intervals. No significant differences were found between the 2 groups of subjects in the masseter motor potentials evoked by transcranial magnetic stimulation. Conclusion: Although the data were only obtained during wakefulness in patients self-reporting signs and symptoms suggestive of SB, the findings suggest that an abnormal excitability of the central jaw motor pathways may be present in SB subjects. This increased excitability could derive from an impaired modulation of brainstem inhibitory circuits and not from altered cortical mechanisms. These results support the view that bruxism is mainly centrally mediated and that it involves subcortical structures. The study also indicates that use of the MIR elicited by the double-shock technique could be valuable in the evaluation of bruxism. Keywords: bruxism, excitability, masseter inhibitory reflex, masticatory system, neurophysiology