Effec of an i.p. injection of diazepam on the jaw opening reflex evoked by electrical stimuli applied to the tooth pulp innervated by mainly high threshold afferents and the lower lip innervated by low threshold afferents was investigated using Wistar albino rats. Small doses of diazepam, less than 2.5 mg/kg, suppressed to the same degree the jaw opening reflexes evoked by stimulation of tooth pulp or lower lip. However, when larger doses thab 5.0 mg/kg were administered, the suppression of tooth pulp-evoked jaw opening reflex was greater than that of lower lip-evoked one. In this case, the period of suppression of tooth pulp-evoked jaw opening reflex was longer than that of lower lip-evoked one. It is concluded that (1) the pain relieving effect of diazepam appears only when larger doses tha 5.0 mg/kg are administered and that (2) the pain-relieving effect lasts longer than muscle-relaxative effect of diazepam.