This presentation is limited to the antifertility effect of the IUD in ewes and to some of the changes in uterine function caused by its presence. Some of these changes may occur only in sheep. A custom-made plastic spiral, 5 cm long and 5-8 mm outside diameter, was inserted into the lumen of only 1 uterine horn. Ovum fertility occurred normally in control ewes but the fertility rate of IUD-wearing ewes was very low, as determined by ovum cleavage at 3 days after breeding. None of the 17 ova was cleaved when ovulation occurred on the spiral side and only 1 in 10 was cleaved when ovulation occurred opposite the spiral. Accessory sperm were not found in the zona pellucidae of any ovum in IUD-carrying ewes while in controls accessory sperm ranged from 10 to over 60. These findings indicate the IUD did more than interfere physically with the passage of sperm up the reproductive tract. The ova was found not defective nor was ovulation interfered with. Myometrial tissue removed and tested in vitro showed adequate contractility. However an altered motility pattern in test animals was shown by in vivo observations of exteriorized uteri. This might partly account for the absence of sperm from the oviducts. Sperm cells placed into the uterine horns were found to be broken into heads and tails or to have disappeared after 5 hours. This effect was markedly greater in horns containing the IUD. Some sperm cells were observed being phagocytized by leukocytes. Spermicidal activity in excised and washed uterine horns could be observed as early as 1 hour after semen injection. This activity was largely stopped when total uteri were excised. Phagocytosis of sperm cells was increased when an inflammatory reaction was producted by bacterial inoculation. Iv dye injections showed resulting dye concentrations directly beneath the turns of the spiral (p less than .01), which was considered to indicate the degree of leakage from the endometrial vascular system. Other experiments showed an IUD intensifies bacterial capabilities of the uterus by increasing the inflammatory response to the presence of foreign substances, whether semen or bacteria. It was concluded that as IUD in the sheep uterus inhibits sperm transport and ovum fertilization, perhaps by changing the nature of uterine motility.