OBJECTIVE: The purpose of this study was to determine whether stretching and shortening of human myometrial strips alters prostaglandin production. STUDY DESIGN: We measured 6-keto prostaglandin F1alpha (PGF1alpha; a PGI2 metabolite) and PGF2alpha in the contraction baths of myometrial strips that were obtained at cesarean delivery from 5 term pregnant women. Fluid was collected after the stretching and after the strip lengths were decreased by 4%, 6%, or 8%. Prostaglandin concentrations were measured in triplicate by competitive enzyme immunoassay and compared by Kruskal-Wallis tests. RESULTS: Stretching myometrial strips increased the production of 6-keto PGF1alpha by 31%, but not PGF2alpha. Shortening strips had little effect on 6-keto PGF1alpha production, but increased PGF2alpha production by 158%. CONCLUSION: Stretching myometrial strips from pregnant women increases synthesis of PGI2; shortening strips increases synthesis of PGF2alpha. Differential prostaglandin synthesis that is related to myometrial stretching and shortening could play a role in uterine quiescence during gestation and increased contractility during parturition.