to reflect the general state of health and fitness and it is conceivable that the heavier (i.e., the healthier} animals, might be more resistant to a given radiation insult. If the comparison is among different strains and on maturing animals, given a more or less uniform state of health, the data of Holland and Mitchell [H10] would be compatible with the hypothesis that body weight might be a measure of the rate of maturation and it would not be inconceivable under these conditions that body weight could be a parameter better correlated with radiation resistance than chronological age. It could also be further sug- gested, as the authors do, that some of the strain-specific differences in radiation sensitivity already discussed in paragraphs 238-244 might be due to differences in the rate of maturation of the various strains or to other nonspecified processes which are in turn highly correlated with maturation rate. C. 1. 256. AGE AT IRRADIATION Irradiation in utero The available dataon shortening induced by in utero irradiation will first be examined. According to Nash and Gowen [N9] who evaluated in multi- factorial experiments the life-spans of 647 mice irradiated in utero with exposures of 20 - 320 R at four different gestational ages, the reduction in longevity induced by the radiation treatment depends on the genetic constitution and sex, as well as on the dose and on the gestational age at irradiation. Rugh, Duhamel et al. [R6] followed the long-term survival of mice irradiated with mid-lethal in utero doses at 0 to 5 days post-conception (p.c.) and found no modification of the life-span in animals that survived later than 30 days post-partum. This observation would indicate that after the neonatal period there is little permanent damage of the in utero irradiation. 257. Non-inbred male and female RF mice were exposed to x-irradiation in utero at ages ranging from 9.5 days p.c. to 1 year of extra-uterine age. Shor- tening of life-span by doses of 50 - 400 R was more effective per unit dose at the higher dose levels of 300 - 400 R and in age groups of 40-70 days of age. For exposure in utero life-shortening was consistently less marked than for exposure after birth, but irradiation with high doses in utero (300 R at 14.5 days p.c.) gave rise to growth and developmental effects in a very high percentage of the animals (particularly males) which took them to death in 6 to { months for causes which were not clear: signs suggestive of a polyarteritis