Female rats had one oviduct, or the cranial tip of one or both uterine horns, lesioned by coagulation, or separated from the remainder of the uterus. After recovery and return to oestrous cycles, myométrial activity at oestrus was analysed by video-laparoscopy. Lesioning the oviduct had no effect on myométrial activity. Coagulating the cranial tip of one horn initially reduced the frequency of ipsilateral longitudinal contractions propagating caudally, but this returned to normal after 14 days. Separating the cranial tip of the uterus had permanent effects on myométrial activity. Separation of one tip reduced the frequency of ipsilateral longitudinal contractions propagating caudally, had no effect on ipsilateral contractions propagating cranially, but reduced the frequency of contralateral contractions propagating cranially. The effect of a lesion near one uterotubal junction on contractions originating contralaterally near the cervix results from communication between uterine horns at the cervical junction; arrival of a caudally propagating contraction in one horn frequently generates a cranially propagating contraction in the other. Separating both uterine tips reduced the frequency of longitudinal contractions propagating in both directions. We conclude that, at oestrus, most spontaneous myométrial contractions are generated by pace-makers in the cranial tip of each uterine horn. The pace-makers are close to the uterotubal junction and regenerate after destruction by coagulation. When their influence is permanently removed, new pace-makers do not develop in myometrium caudal to the lesion. We conclude that most myométrial cells do not exhibit spontaneous pace-maker activity in vivo.