Rats were irradiated after 10 days of gestation with 20 R (10 20 group) or 100 R (<tex-math>$10^{100}$</tex-math> group), or after 15 days of gestation with 50 R (<tex-math>$15^{50}$</tex-math> group) or 185 R (<tex-math>$15^{185}$</tex-math> group). The incidence of still-birth was increased only in the <tex-math>$15^{185}$</tex-math> group while neonatal mortality was elevated in this and the <tex-math>$10^{100}$</tex-math> groups. Birth weight was reduced in all groups other than the 10 20 but growth was depressed only in the rats irradiated after 15 days of gestation. The relative growth of a number of organs, particularly the brain and spleen, was further reduced in these animals. The mortality of the rats of the <tex-math>$15^{185}$</tex-math> group was most markedly altered although lesser changes were seen in the <tex-math>$10^{100}$</tex-math> and <tex-math>$15^{50}$</tex-math> groups; the effect on the females was greater than on the males.

Rats of various ages were exposed to graded levels of 131 I, and randomly selected animals were sacrificed after 1, 3, or 7 days to determine radiation doses to the thyroid glands. The doses were found to vary with age and were not a linear function of administered dose. After four additional months, the remaining rats were injected with a tracer dose of 131 I and sacrificed 24 hours later. Radiosensitivity was expressed as the initial radiation dose, in rads, required to reduce incorporation of the tracer to 50% of control levels. On this basis, values of 18,000, 12,000, 5300, and 970 rads were obtained for the initial exposure of adult, weanling, newborn, and prenatal rats, respectively. Alterations in growth and in extrathyroidal iodine metabolism were also noted in these animals. These effects varied with age and dose, but it was not established whether they were direct radiation effects or secondary to the induced hypothyroidism.