The reflex responses evoked by controlled mechanical stimulation of an upper central incisor tooth in single motor units in the human masseter muscle were examined. The stimuli were (brisk) taps and (slow) pushes of about 2 N peak force, applied orthogonally to the labial surface of the ipsilateral upper central incisor tooth. The reflex responses of the motor units were characterised by analysis of the changes in the durations of the first and second interspike intervals (ISIs) immediately following the stimulus. A significant increase in the duration of these ISIs in comparison with pre-stimulus ISIs indicated inhibition, and significant shortening indicated excitation. Twenty masseter motor units were tested with both the pushes and the taps. The brisk taps elicited a significant reflex inhibition in 16 of the 20 motor units at a latency of 13 ms and duration of 37 ms. This inhibition was followed by significant excitation in 11 of the 20 units at latencies of 71 ms, lasting for 29 ms. The short-latency response to slow pushes was significant inhibition in four units: significant excitation in one unit and no response in 15 units. The slow pushes evoked a significant long-latency excitatory reflex response in 12 of the 20 units at latencies of 77 ms and lasting for 40 ms. The shapes and amplitudes of the compound post-synaptic potentials underlying the reflex responses in the motoneurones were estimated. It is concluded that stimulation of periodontal mechanoreceptors usually activates an excitatory reflex pathway to the jaw-closing motoneurones. This probably helps to grip the food bolus between the teeth during chewing. However, when the rate of application of the stimulus is large enough, a short-latency inhibitory response is evoked which, if of sufficient duration, may over-ride the subsequent excitatory response. Inhibition of the jaw-closing muscles will tend to protect the teeth and soft tissues when one bites unexpectedly on a hard object while chewing.