deleterious effect of irradiation long-term survival data should first be corrected for diseases known to be induced specifically by radiation. The concept of life-shortening appears ambiguous in that it may be regarded either as an overall measure of deleterious effects or as a measure of the effects remaining after allowance for the induction of tumours and other defined diseases. Non-specific life-shortening, if it exists, must recognize some basis in damage to biological structures and functions. Actually, some damage, anatomical or functional, may indeed be shown for some tissues [Ch, D3, C5!, but not for all, and under these conditions an increase of causes of death related to damage in those particular tissues might be expected. The resulting effect should be a change in the spectrum of diseases induced in irradiated animals. On the other hand, one could envisage that damage to body components like the blood vessels or the connective tissue, which are uniformly distributed in the body, could be the cause of non-specific lifeshortening. But here again it would appear unlikely that damage to blood vessels in skin, muscle and fat tissue might result in changes ultimately affecting the vital capacity of an individual, whereas damage to blood vessels of the kidney, the brain, the heart would be expected to result in important changes affecting the capacity for survival. Thus, if the determining fac- tor would be the anatomical location of the damaged blood vessels, again a change in the spectrum of diseases would be expected in irradiated animals, compared to unexposed ones. 38. In conclusion, the notion of non-specific life-shortening is super- imposable with that of aging (advanced or accelerated) induced by radiation. The two concepts are inevitably linked with the demonstration that radiation, although advancing the instant of death, should not modify the spectrum of induced diseases. probably This demonstration is not only difficult in practice but impossible to visualize conceptually since the mechanisms that might be hypothesized for non-specific effects, would also change the spectrum of diseases appearing in irradiated animals. Such change would in itself be in- compatible with the notion of aging or of non-specific life-shortening. 2. 39. The notion of aging Attempts to show that irradiation may non-specifically age animals were started by observations that radiation did produce life-shortening and that irradiated animals showed phenomena similar to those observed in old age