Background: Recently, it has been demonstrated that endothelin(ET) and endothelin related peptides are present in the blood and plasma ET levels are increased after operation. But the causes of increasing plasma ET levels are not clearly understood. This current study was to investigate the relationship between postoperative pain and endothelin. Methods: Thirty adult patients, scheduled for upper abdominal operation under general anesthesia, were included. After operation, epidural catheterization was done for postoperative analgesia. Before induction, on complained of pain and 1 hour after analgesics administration, blood samples were obtained to measure plasma ET levels. Plasma ET concentration was determined by radioimmunoassay. Pain score was measured by visual analogue score(VAS). Mean arterial pressure(MAP) and heart rate(HR) were also recorded every sampling time. Results: There were no significant changes in plasma ET levels at the time before induction versus at the time of the pain complaints and at 1 hour after analgesic administration. Pain score was significantly reduced after epidural analgesia. There was no significant correlations between pain score and plasma ET levels. There were no significant correlation between plasma ET levels and either MAP or HR. Conclusions: These results indicate that there is lack of relationship between postoperative pain and endothelin.