Continuous-wave Doppler ultrasound was used to assess blood flow velocities and determine the pulsatility index of the uterine arteries in ten pregnant women at 23-33 weeks' gestation during indomethacin therapy for preterm labor or polyhydramnios. There was no statistically significant difference in the pulsatility index of the uterine arteries during maternal indomethacin treatment (mean +/- SD 0.85 +/- 0.29) compared with pre-treatment values (0.85 +/- 0.23). Although studies in animals have reported that indomethacin increases uterine impedance to flow, it appears that it does not affect the uterine impedance in humans.