The effects of various intra-uterine devices on pregnancy in the rabbit were recently described by us (Adams & Eckstein, 1964, 1965). All of them produced increased prenatal mortality, both before and after implantation. While it was established that the devices exerted their contraceptive effect after the eggs entered the uterus, no insight into the more specific aspects of their action was obtained. That it may be complex is indicated by the finding reported below, that two minor variants of the same device, differing only in their mode of attachment to the uterine wall, may produce markedly different effects.Our observations are based on eighteen pregnancies in sixteen does, five nulliparous and eleven primiparous, all of which were included in our earlier study (Adams & Eckstein, 1965). At laparotomy,
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