SUMMARY In the mouse, a unilateral intra-uterine device (IUD) exerted a bilateral contraceptive effect by causing the elimination of embryos from both uterine horns and the bilateral inhibition of the decidual cell reaction. Both uterine horns showed leucocytic infiltration into the endometrium and uterine lumen. The bilateral effect was correlated with the existence of luminal continuity between the two uterine horns. Conception proceeded normally in the control uterine horn that had been completely separated from the IUD horn. An IUD did not usually disturb the function of the Fallopian tube, but tube-locking of embryos occasionally occurred as an artifact. The contraceptive effects of an IUD were exerted within the uterus during the period immediately before implantation.