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Groups of pregnant mice were irradiated at selected times between 10.00 hours on gestation day 7 and 16.00 hours on day 8. Each group received 0.39 Gy of neutrons or 1.60 Gy of X-rays, or was sham irradiated. We identified a period of high susceptibility of the embryos to radiation-induced exencephalia, anophthalmia and prenatal mortality early in gestation day 8. Dose-incidence relationships in this period were investigated with 0.19-0.48 Gy of neutrons and with 0.40-2.00 Gy of X-rays. With increasing neutron dose, incidence of exencephalia in live embryos rose and then declined. This response suggests that embryos with neutron injury of the type that leads to exencephalia are at a greater risk of dying in utero than are similarly irradiated embryos not so injured, and that this risk increases with dose. A model is proposed that accounts for the shape of the neutron dose-incidence curve. X-ray-induced exencephalia showed only an increase with dose. In X-irradiated litters, almost invariably, the incidence of anophthalmia was higher in exencephalic than in nonexencephalic embryos and the ratio of these incidences (relative risk) decreased toward 1 with increasing dose. A model is proposed that accounts for these observations. The incidence of bilateral anophthalmia in X-irradiated embryos was higher than would be expected if the bilateral form resulted solely from independent injury at each of two equally susceptible sites.

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