Effect of electro-acupuncture (EA) on the jaw opening reflex induced by tooth pulp stimulation was investigated in Wistar albino rats. The amplitude of the digastric muscle EMG (dEMG) evoked by stimulation of the lower incisor tooth pulp was measured as the magnitude of the reflex and estimated as an indicator of the EA effect. Acupuncture needling was given at one meridian point of Ho-Ku and two other nonmeridian points in one forepaw. Compound action potentials of ulnar, median and radial nerves elicited by 15 min EA stimulation at 45 Hz through the needling points were recorded from the brachial plexus innervating the same acupunctured forepaw. Of the above-mentioned three nerves, the afferent volleys in the radial nerve were found to be the most effective to suppress the dEMG during EA stimulation. The EA stimulation of the Ho-Ku point showed a greater effect than that of nonmeridian points. There was a significant correlation between the amplitude of Abeta fibers in the compound action potentials elicited by EA stimulation and the dEMG suppression. To confirm the so-called morphine-like action of EA that has recently been proposed, levallorphan (LORFAN), antagonist of morphine, was administered before and after EA stimulation. In the premedicated case, no suppression of the dEMG occurred. In the postmedicated case, on the contrary, the dEMG responses which were reduced during 15 min EA stimulation recovered more rapidly than in the case without LORFAN. These results suggest that the afferent impulses conducted along Abeta fibers mainly in the radial nerve cause the release of endogeneous morphine-like substances in specific central nervous areas for suppressing the dEMG activity.