OBJECTIVE To clarify the effects on uterine arterial flow velocity waveforms of uterine contractions following oxytocin infusion and during spontaneous labor. METHODS Uterine arterial flow velocity waveforms were obtained by pulsed Doppler methods from 22 women during an oxytocin challenge test (OCT), 26 women during oxytocin-induced labor, and 40 women during spontaneous labor. Mean resistance index (RI) for bilateral arteries was used for analyses. After the onset of labor, flow velocity waveforms were assessed according to cervical dilatation. During OCT, Doppler flow velocimetry was performed when three uterine contractions occurred per 10-min period. RESULTS RI values did not differ significantly between induced and spontaneous labor during relaxations at any level of cervical dilatation. However, during contractions, RI was significantly higher for induced labor than for spontaneous labor. Absence or reversal of flow was more frequent in the OCT group than in the induced labor group (P < 0.0001). However, no significant differences were found between spontaneous and induced labor groups. CONCLUSIONS Interactions between the contracting uterine body and the relaxing lower segment in oxytocin-induced labor might be associated with differences in uterine arterial flow during contraction between oxytocin-induced and spontaneous labor. However, changes in the intensity of uterine contractions during labor progression might differ between oxytocin-induced and spontaneous labor.