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 F1α (PGF1α; a PGI2 metabolite) and PGF2α 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 PGF1α by 31%, but not PGF2α. Shortening strips had little effect on 6-keto PGF1α production, but increased PGF2α production by 158%. Conclusion: Stretching myometrial strips from pregnant women increases synthesis of PGI2; shortening strips increases synthesis of PGF2α. 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. © 2008 Mosby, Inc. All rights reserved.