Pregnant squirrel monkeys received tritiated water (HTO) in the drinking water throughout gestation at levels ranging from 16 to 1000 times the permissible level for human consumption (0.003 μCi/ml), resulting in mean body water HTO levels ranging from 0.05 to 3.1 μCi/ml. There were no discernible effects of HTO administration on the newborn progeny in terms of body weight, body dimensions, selected organ weights (brain, heart, adrenal, kidney, liver, spleen), hematologic patterns, and histology of selected organs and tissues (adrenal, kidney, liver, lung, brain, pancreas, jejunum, pituitary, spleen, testes, thymus, skin) other than ovaries. The number of primary oocytes in female progeny decreased markedly with increasing levels of HTO in maternal drinking water. Quantitative analysis of neonate ovaries, testes, brain tissue, and retinal tissue is in progress. No effects of HTO administration on maternal body weight, gestation time, or maintenance of pregnancy to full term were observed. Body weights of HTO-treated inseminated females that did not deliver were less than control weights, but the lack of dose dependence implies that this effect may have been associated with a stimulus characteristic of the HTO administration rather than with irradiation.

Mice were exposed to gamma radiation from 60 Co sources at 1750 R/h and at 190 R/h. The 30 day LD 50 was 873 R at 1750 R/h and 1359 R at 190 R/h. Survival curves of bone-marrow colony-forming cells measured between 100 and 500 R had values of D 0 of 90.1 R for the 1750 R/h exposure rate and 92.9 R for the 190 R/h exposure rate. The corresponding extrapolation numbers of the survival curves were 1.93 and 1.83. It is concluded that a difference in rate of exposure to radiation sufficient to result in a large difference in the LD 50 is not necessarily associated with a corresponding difference in response of the bone-marrow colony-forming cells to the radiation.

The LD 50 in mice due to gastrointestinal radiation injury from exposure to either 60 Co gamma rays or 250 kVp x-rays has been measured for exposure rates ranging from over 100 R/minute down to less than 1 R/minute. The LD 50 for 60 Co gamma rays was greater than the LD 50 for x-rays, but at all exposure rates there was a nearly constant ratio of LD 50 for the two kinds of radiation. Measurements of rate of recovery of radiation injury were made with x-rays, using the split-dose technique. The recovery was exponential, with a half-time of 23 minutes. The amount of repairable injury was about half of the exposure given for exposures up to 470 R, but became nearly constant and independent of exposure between 470 and 700 R. A model for repair of injury during exposure was constructed on the basis of the measured recovery kinetics. The increase of LD 50 with decreasing exposure rate of either type of radiation was shown to be reasonably accounted for by the model for repair of radiation injury.

The destruction of erythropoietic stem cells by irradiation with fission neutrons was studied in plethoric C57 leaden male mice by measuring the incorporation of Fe 59 into erythrocytes 48 hours after a dose of erythropoietin. The survival of stem cells in mice exposed to single doses of neutrons at 37 rads/min followed the single-hit multitarget model of radiation injury to cells: $S/S_{0}=1-[1-{\rm exp}(-D/D_{0})]^{E}$ , where $S/S_{0}$ is survival fraction, D is dose, D 0 is 37% dose, and E is target multiplicity. The value of D 0 was a constant, equal to 27.8 rads for all animals, but E varied from 1 to 4 different animals. The survival of stem cells in mice exposed to single doses of neutrons at 1.75 rads/min followed the single-hit multitarget model, with $D_{0}=26$ rads, for those animals for which E = 1, but animals with E > 1 did not fit the model. The survival of stem cells in mice exposed to three doses at 37 rads/min at intervals of 10 to 12 hours also followed the single-hit multitarget model, with mean $D_{0}=26$ rads, and a distribution of values of E similar to that in the single dose. The data strongly imply that the loss of stem cells follows the multitarget model, but that E, instead of being a constant, is a basic biological variable of the response.