2. 262. Irradiation during the extra-uterine life Experiments where the effect of the extra-uterine age has been examined as a biological variable affecting radiation-induced life-span-shortening are fairly numerous and cover a variety of different species, strains and conditions of irradiation. 263. Kallman and Kohn [K7], Kohn, Kallman and Berdijs [K1] and Kohn and Gutt- man [{K6, K14] studied the life-shortening response of x rays of male and female CAF1 mice, with special regard to the influence of age at irradiation. Expo- sure conditions were: 250 kVp x rays at a dose-rate of 40 - 45 rad/min with two single doses of 260 or 520 rad and a fractionated dose of 260 x 2 rad given 8 days apart. Mice were irradiated when young (144 and 164 days of age) or old (385, 550 and 730 days) and followed with detailed pathology at death. The age interval covered in the mouse would correspond in man to a range of ages be- tween 18 and 65 years. The last publication of Kohn and Guttman [K6] gives an account of the whole set of data, including a reanalysis of other previous data obtained on Balb/c mice irradiated at 5 months, 1.2 or 1.4 years of age. In ge- neral, some reduction of the life-span was observed, although in some cases evi- dence of life-shortening was small or even absent. 264. Old adult mice tended to show less life-shortening than young ones, al- though the difference was not the same in all strains used owing to genetic differences. During much of the adult life the female animals were more sensitive to doses below 250 rad. Later in life, however, the females, at least in the CAF1 strain, became less sensitive than the males. The life-shortening in older animals was not associated (as in younger ones) with an increased induction of neoplasia, but rather with a decrease. In the CAF1 mice irradiation tended in fact to reduce the number of animals dying with tumours and the tumour-—bearing animals lived as long or longer than the non-tumorous ones. Aging (both pre- mature or accelerated) was as such an inadequate explanation for these data because the irradiated animals appeared to age abnormally and usually, but not necessarily, died sooner than controls, the effect depending on age at exposure, sex and dose [K6]. 265. Boone et al. [B19, B20] on CF1 female mice at ages from 1 day to 18 months reported changes in life-shortening as a function of age at exposure. More pre- cisely, after 400 rad of x rays life-shortening amounted to 40 per cent of con-