Two hundred and eighteen female rats were utilized in this study to evaluate the indirect effect of maternal x-irradiation on embryonic development and survival. There were two control groups and ten experimental groups receiving 0, 60, 150, 275, or 400 R on the afternoon of the first day. The use of special shielding devices permitted irradiation of either the mother or the zygote. Several groups of pregnant rats were exposed to 150 R on the afternoon of the first day of gestation. Irradiation of the oviduct and the ova resulted in a 60 to 70% resorption of the implanted ova. Irradiation of the maternal organism while the oviduct was shielded did not increase the resorption rate above control levels. Thus, no indirect effect was demonstrated when the maternal organism received 150 R. When the maternal rats received 400 R and the ova were shielded, 25% of the implanted embryos resorbed. This was apparently not due to leakage of irradiation around or through the special shielding. Thus, high doses of irradiation in the rat have a moderate indirect effect, but this indirect effect first becomes manifest when the dose of irradiation is above the <tex-math>${\rm LD}_{90}$</tex-math> following direct irradiation of the ova. Neither maternal irradiation nor zygote irradiation on the first day of gestation results in an increase in gross congenital malformations or produces fetal growth retardation.

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