Objective The possibility exists that the vehicle for 17-alpha-hydroxyprogesterone caproate, castor oil, exerts an effect on human uterine contractility. The aim of this study was to evaluate its effects on contractility of myometrial preparations that were obtained during pregnancy. Study Design Myometrial strips were suspended under isometric conditions. Contractility was induced with oxytocin. Strips were incubated in castor oil or physiologic salt solution and suspended for a further oxytocin challenge. Contractile integrals were compared between both groups. Results Strips that were exposed to castor oil demonstrated increased contractile activity that was elicited by oxytocin (mean contractility value, 165.53% ± 17.03%; n = 8; P = .004), compared with control strips (mean contractility value, 72.57% ± 7.48%; n = 8; P = .003). There was a significant increase in contractile activity of the castor oil–exposed strips, compared with those that were exposed to physiologic salt solution (n = 8; P < .001). Conclusion Exposure of human myometrial preparations to castor oil results in enhanced oxytocin-induced contractility.