The oxytocin-inactivating activity (OIA) of rat uterus homogenates was studied in intact animals and in animals with bilateral intrauterine devices (IUD). In another series of experiments the OIA of the rat uterus was related to the mast cell count of uteri from intact rats and from rats with bilateral intrauterine devices. The OIA of the homogenates was significantly higher at oestrus than at dioestrus. No such increase was observed in homogenates from rats at oestrus with bilateral IUD's. The IUD caused an increase in the mast cell population at oestrus and the increase in mast cell counts observed in uterine homogenates of intact rats at dioestrus was not observed in the presence of a device. No correlation between the OIA of uterine homogenates and the mast cell population was observed in animals with or without IUD at oestrus or dioestrus.