OBJECTIVE: The methodology used to evaluate contractile effects of uterotonic agents in human myometrium in vitro varies. The are no studies evaluating the reliability of these commonly used techniques.STUDY DESIGN: Myometrial strips (n = 72) were exposed to 3 known uterotonic agents: oxytocin, U46619, and phenylephrine. The negative log of the molar concentration of the agonist that produces a half-maximal response (pEC(50)) and maximal response values were obtained, and compared, when either amplitude or mean force was used as indices of contraction. All data were expressed as a percentage of KCl elicited actvity.RESULTS: Using pEC(50) measurements, the order of potency was oxytocin greater than U46619 greater than phenylephrine for both indices, whereas the order of maximal response varied between mean force and amplitude. The coefficient of variation was lowest for pEC(50) measurements, highest for maximal force estimations, and overall was 10-48% between, and 2-27% within, donor samples.CONCLUSION: These findings support the use of pEC(50) measurements for in vitro experiments using uterotonic agents and outline the variability that occurs for such myometrial experiments.