The purpose of this study was finding the pain inhibitory effect of acupuncture based on rataining time at LI -4. The pain at dentes incisor was evoked by noxious electric stimulation and digastric electromyogram(dEMG) changes based on time interval were measured. To do this, the opioid antagonist was administered intraperitoneally and four groups were made for convenience. Without naloxone, dEMG was changed by either retaining the needle for 40 minutes (Group I) or by lifting and thrusting the needle (Group II). With naloxone administration, dEMG was changed by either retaining the needle for 40 minutes (Group III) or by lifting and thrusting the needle (Group IV). The results are as following 1. The pain inhibitory effect of acupuncture at LI -4 was expressed best in Group I. 2. The pain inhibitory effect was somewhat expressed in Group II but the effect was smaller than Group I. 3 .In Groups III and IV, the pain inhibitory effect was not expressed. The overall result should be the foundation for the further studies to figure out the underlying mechanism of acupuncture. In addition, it is assumed that the results will be useful for optimal retaining time of acupucture for its maximal effect.