Uterine contractile activity was monitored in pregnant and parturient rats by means of previously inserted intrauterine balloons before and during oxytocin infusions. Labor could be induced at term by an infusion of 4-hr duration (1 mU/min for 2 hr followed by 2 mU/min for 2 hr). The duration and outcome of oxytocin-induced labor were very similar to spontaneous labor, and the oxytocin-induced contractions could not be distinguished from spontaneous contractions.