Background: Intravenous anesthetics are often used for conscious sedation in endoscopic retrograde cholangiopancreatography (ERCP) and endoscopic sphincter of Oddi (SO) manometry. This study was designed to investigate the direct effects of some intravenous anesthetics on SO in sheep. Methods: In sheep SO rings, changes in isometric tension in response to cumulative concentrations of intravenous anesthetics were determined, and values for E-max (mean maximal inhibition) and pD(2) (i.e. the negative logarithm of the concentration for the half-maximal response, EC50) were compared. Results: Meperidine (10(-)7 to 3x10(-5) M), fentanyl (10(-7) to 3x10(-5) M), midazolam (10(-7) to 3x10(-5) M) and propofol (10(-7) to 3x10(-4) M) induced concentration-dependent relaxations on SO precontracted with carbachol (10(-6) M). E-max and pD(2) values following meperidine, fentanyl and midazolam administration were significantly greater than after propofol (p<0.05). There were no significant differences in E-max and pD(2) values for meperidine, fentanyl and midazolam. Conclusion: These results suggest that meperidine, fentanyl and midazolam are equipotent relaxants in the sheep SO in vitro. The relaxatory effect of propofol was 10 times less potent compared to the above agents, and it can be beneficial during SO manometry in controlled clinical human studies. Copyright (C) 2005 S. Karger AG, Basel and IAP.