OBJECTIVE: The purpose of this study was to determine whether shortening alters spontaneous contractility in myometrial strips that are obtained from pregnant women. STUDY DESIGN: Isometric contractions were measured in myometrial strips that were obtained at cesarean delivery from 14 pregnant women at term. After 2 hours of stretching, the strip lengths were decreased by 4%, 6%, or 8%. Spontaneous contractility was measured for 120 minutes with or without prostaglandin synthase inhibitor indomethacin (10 -5 mol/L), and the cumulative concentration response to oxytocin was determined. RESULTS: Contractility was increased by 29% and 34% in strips that shortened by 4% and 6%, respectively. Preincubation with indomethacin increased contractility by 15% in stretched strips and decreased contractility by 30% and 19% in 4% and 6% strips, respectively. Contraction frequency was increased by 26% and 53% for the strips that were shortened to 6% and 8%, respectively. These increases were prevented by indomethacin. The oxytocin responses were similar at all lengths. CONCLUSION: Shortening of myometrial strips from pregnant women at term increases spontaneous contractility by a mechanism that apparently involves prostaglandins.