Although several studies have been conducted regarding the jaw-jerk reflex and its various modifying parameters, the effect of intercuspal clenching still remains poorly understood. Evidence of both excitatory and inhibitory influences of periodontal receptors on jaw muscle activity has been reported. It is also suggested that a level of background activity in the stimulated muscle facilitates the reflex response. It was therefore the purpose of this study to investigate the effect of simultaneously activated periodontal receptors on the various parameters of the masseteric jaw-jerk reflex during moderate masseter muscle activation. Electromyographic recordings were obtained from the masseter muscle of the preferred chewing side at rest and at intercuspal clenching in 10 men, by use of a computerized recording and analysis system. In the clenching trials the level of muscular activity was set at 40% of the maximum value obtained through visual feedback. The results showed that at 40% clenching the occurrence of the reflex was reduced as compared with that obtained at rest. Moreover, when present during clenching, its latency and duration were shorter, whereas its amplitude was higher. It was suggested that the simultaneous activation of periodontal receptors may occasionally inhibit the masseteric monosynaptic excitation.