In order to investigate myotatic reflex involvement in jaw muscle control, an analysis was made of the motor responses induced by mechanical vibration (120-160 Hz) of the jaw elevator muscles in healthy subjects. As seen in torque measurements and mean-voltage electromyographic (EMG) recordings, the vibration caused involuntary reciprocal changes in jaw muscle tone, the contraction force increasing in jaw elevators and decreasing in antagonistic jaw opening muscles. This tonic vibration reflex (TVR) elicited from the jaw elevators exhibited many characteristics similar to those previously described for limb muscle tonic vibration reflexes: it varied in strength from one subject to the next independently of the briskness of the jaw elevator tendon jerks; it had a gradual onset with successive recruitment of jaw elevator motor units firing largely out of phase with one another and at rates much lower than the vibration frequency; it was susceptible to voluntary control--when allowed visual feed-back from the torque meter all subjects were able to suppress the TVR and keep mean contraction force constant. The results indicate that with respect to the tonic motor response to sustained inflow in the Ia afferent nerve fibres, the jaw elevators do not differ markedly from other skeletal muscles. Independently of whether a TVR was present or not, the vibration caused a timing of the motor unit discharges in the jaw elevators that could not be controlled voluntarily and that showed up in gross EMG recordings as a marked grouping of discharges synchronous with each wave of vibration. A similar but less distinct grouping of the gross EMG pattern was seen in limb muscles exposed to vibration, the dispersion increasing with the peripheral conduction distances of the reflex arcs. It is suggested that contrary to the TVR, which depends on the sustained mean level of the Ia afferent input, the timing phenomenon depends, like the tendon jerk, on the degree of synchrony in the afferent Ia volleys. Monosynaptic projections may well be involved in the dynamic timing of motor discharges during tonic firing, but this does not imply that the TVR or the tonic stretch reflex is dependent upon such projections.