Jaw opening reflex (JOR) induced by tooth pulp stimulation and re- corded electromyographically from the ipsilateral digastric muscle was studied on 61 rabbits under light anesthesia. Electro-stimulation of ipsila- teral superficial radial nerve initiated an immediate and marked suppres- sion of this reflex which showed two phases: 1. Early phase

the JOR was suppressed as soon as the electro-stimula- tion was started which attained its maximal effect within half a minute and persisted for 2.5-5 minutes. This effect could not be reversed by na- loxone

but was abolished after decerebration rostral to superior colliculi of midbrain. 2. Late phase

prolonged moderate suppression of JOR was noticed under continuous electrostimulation which persisted even after cessation of stimulation for 5 minutes. This effect could be partially reversed by naloxone and was abolished after ablation of diencephalon. It is thus concluded that the early phase of the suppressive effect on tooth pulp induced JOR by electroacupuncture involves the central mechanism located rostral to the superior colliculi of midbrain

while its late phase is mediated through diencephalon in which the endogenous morphine like sub- stance might be involved.