The intra-uterine balloon is one of the commonest methods used in investigating uterine contractility. The micro-balloon measures the maximum intra-uterine pressure that is developed in a closed system as the myometrium contracts. In rats which had an intra-uterine micro-balloon, parturition progressed normally and no stillbirths were observed. The average time for each fetus to be delivered, 9.4 .+-. 0.7 min, was significantly longer (P < 0.05) than the control group and sham-operated groups. The duration of the parturient period in this group was 96.5 .+-. 8.7 min which was significantly longer (P < 0.05) than the control and sham-operated groups. The parturient period in the balloon group was prolonged uniformly in the individual rats, the prolongation not being due to fetuses being mechanically impeded by the balloon or catheter.