diation was simulated by a displacement of the Gompertz function on the time axis. Johnson was thus able to show that a decrease in the life~shortening ef- fect with increasing age was a necessary consequence of the hypothesis that ionizing radiation accelerates certain of the processes that characterize natural aging. The relationship of life-shortening to age at irradiation varies however according to the actual form of the Gompertz function. 271. Age-dependent changes in the response to radiation (250 kVp x rays) were also observed in other experiments by Cosgrove et al. [C20] where LAF1 mice were given 300 to 1200 R whole-body or partial-body when 10 weeks or 1 year old. At any given exposure level, a higher incidence of glomerulosclerosis was observed in animals irradiated at the younger age, presumably because the older animals did not survive long enough to develop as high an incidence of the diseases. Longevity was also reduced and the incidence of ovarian tumours increased in the young but not in the old irradiated animals. 272. Some data by Storer [S20] in mice are of interest to the problem under discussion. BDF1 females, when three months old, were exposed at doses of Q, 100, 300, 500 R of 250 kVp x rays. Median survival time was found to be re- duced linearly with dose and the slope of the linear non-threshold regression function amounted to a life-span reduction of 45 days/100 R. The changes in radiation response with advancing age and for various radiation doses were evaluated in two ways: beginning at various ages, samples of previously-irra- diated surviving animals were tested for their ability to survive successive daily exposure of 100 R; 273. or, alternatively, they were tested for their EDS 9° Thirty-four samples of previously-exposed animals were tested at ages ranging from 120 to 960 days with 100 R/day and resistance was assessed as mean survival time after initiation of this treatment. Resistance was found to fol- low a long plateau (the duration of which was dose-dependent) and to decline sharply at advanced ages. Pre-irradiated animals were less resistant than non- irradiated control mice of the same age and showed an earlier onset in the decline of resistance. Since the variability in radiation resistance increased with age and the differences in sensitivity between irradiated groups were reduced when the relevant comparisons were conducted at equal levels of mortality, Storer [S20] concluded that the challenge treatment was not actually measuring a pheonomenon intrinsic to the aging process but was more simply an estimate of