Electromyographic recordings from 12 pregnant sheep were analyzed in four gestational age groups (106 to 115, 116 to 125, 126 to 135, and 136 to 145 days) and by the day of delivery. Mesometrial electromyographic activity was present in all animals studied. Contracture activity was synchronous in both mesometrium and myometrium. The frequency distribution of electromyographic events of different duration in the mesometrium was qualitatively identical to that recorded from the myometrium. The frequency of events less than 180 seconds was greater in the myometrium, while no difference was observed in contracture frequency. Mean contracture duration in myometrium and mesometrium shortened as gestation progressed (p less than 0.01), and was of significantly shorter duration on the day of delivery when compared with all gestational age intervals (p less than 0.05). Except at 135 to 145 days' gestation and at the day of delivery, myometrial contractures were of significantly longer duration than mesometrial contractures. The presence of the endometrium and fetal membranes in the pregnant ewe may stimulate the myometrium, and account for the difference observed. In vivo recording of mesometrial electromyographic activity may provide an opportunity to study the paracrine regulation of myometrial activity.