Sprague-Dawley rats received whole-body exposures of 50-600 rad of cobalt-60 gamma rays or extended-Bragg-peak helium ions on Days 3-10 of pregnancy. Uterine contents were examined on the twentieth day of gestation. In gamma-irradiated litters, LD50 in utero values ranged from 140 rad on Day 3 to 515 rad on Day 6 of gestation. In litters exposed to helium ions, LD50 in utero values ranged from 170 rad on Day 10 to 490 rad on Day 6. The embryos were most resistant to both radiations on Day 6 of gestation (implantation), and most sensitive to both on Days 9 and 10 of gestation (early organogenesis). While the radiosensitivity of the rat embryo fluctuated by a factor of 3-4 between Days 3 and 10 of development, the magnitude of the fluctuation was virtually identical in animals exposed to gamma rays or helium ions, and the RBE of the latter with respect to embryo killing averaged 1.0 during this period of development.

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