SUMMARY: The release of prostaglandin-like material and the spontaneous contractions of individual horns from the pregnant rat uterus in vitro have been studied on day 22 of pregnancy – the expected day of delivery. Removal of foetuses (retaining placentae in utero) from one or both uterine horns on day 16 or 17 significantly reduced prostaglandin F release and spontaneous activity. Rats which had been made unilaterally pregnant after ligation of one uterine horn, exhibited a decrease in prostaglandin F output from both horns. Uterine activity and prostaglandin release were increased in quiescent uteri by the addition of arachidonic acid (5 μg/ml) or phospholipase A (160 mu./ml); these effects were abolished by indomethacin (20 μg/ml). However, the stimulation of uterine activity by PGF2α (30–60 ng/ml) was not affected by indomethacin. It is concluded that the release of prostaglandins from the pregnant rat uterus in vitro at term is related to the presence of viable foetuses.