Copper bearing intrauterine devices have become the most reputable effective and safe IUDs available. The reaction between the copper and the body prevents fertilization. An infiltration of polymorphonuclear leukocytes occur in the uterine lumen as a result of the IUD. The daily released copper is equivalent to 1% of the copper taken orally by day. Alpha amylase activity is depressed; cyclic increase in glycogen synthetase is eliminated. Copper can inhibit enzyme activity even if cupric ions from the metal are at a low concentration. The effect of copper on the myometrium accounts for the lower initial expulsion rates for copper bearing devices. The uterus loses its ability to contract after prolonged exposure to copper. The chemical breakdown between cuprous oxide and saline produces a cuprous complex which yields cupric ion and free radicals. The inhibition of the implantation process of the copper may lead to an explanation. Metallic copper liquifies endometrial mucoids. Cupric ions increase the biosynthesis of certain prostaglandins. Prostaglandins may be another explanation of implantation inhibition. Further research on implantation is needed.