Endothelin-1 (ET-1), known as a vasoconstrictor, induces pain signals through its specific receptor ETA (ETAR). We previously reported that the novel selective ETAR antagonist compound E (provided by Eisai Co., Ltd.) restored the ET-1-induced attenuation of morphine-induced analgesia with HEK293 cells stably expressing both ETAR and µ-opioid receptor (ETAR/MOR cells) and mice model. Further, among several opioids including morphine, we found that ET-1 almost completely suppressed the MOR activity induced by fentanyl (attenuated to 4.8 %) compared to that of morphine (attenuated to 37%), indicating that analgesic effects induced by fentanyl could be more influenced by endogenous ET-1. Several studies have reported that the fentanyl binding site to MOR is different from that of morphine, and these differences may also be involved in the distinct attenuation of MOR activity by ET-1, although the precise mechanism remains unclear. In the present study, in order to analyze the mechanism by which the analgesic attenuation effect of ET-1 differs among opioids, we are in the way to investigate and compare the inhibitory effect of ET-1 on MOR activity by morphine or fentanyl.