Rhythmic contractions occur in the sheep uterus at oestrus and during pregnancy from about 65 days to term (145 days). To define factors responsible for these contractions we have examined and quantified the degree of synchronization of electrical activity in the uterus and isolated segments of myometrium in 3 types of experiments. In the first, a segment of myometrium was totally separated from the uterus. After a period of 9-16 days the isolated tissue developed a typical pattern of uterine activity which showed no significant degree of synchronization with EMG bursts in the body of the uterus. During labour, the isolated tissue showed changes in activity similar to those observed in the uterus. In the 2nd experiment, the tubal end of one of the uterine horns was severed from the uterus, but a connection was retained with the uterus via the oviduct and ovarian blood vessels. Activity in the partly isolated segment remained in synchrony with the uterus. In a 3rd experiment, impulse propagation through nerves and smooth muscle to the tip of a horn was disrupted by severing 'the tip' from the uterus while its blood supply from the ovarian vessels was retained. The blood vessels were momentarily frozen, and denervation confirmed by monoamine histofluorescence. In 5 out of 6 animals the operated tissue showed activity that was not synchronous with the rest of the uterus. These data indicate that: (1) isolated uterine muscle in vivo has rhythmicity resembling that of intact myometrium and (2) systemic or local circulating factors are not responsible for synchronizing uterine activity before parturition, although circulating factors do play a major role in increasing the uterine activity which occurs at parturition and at oestrus, and (3) hydraulic continuity between different regions of the uterus is not essential for maintaining co-ordinated activity.