variables affecting an ill-defined effect such as aging. Pending clarification of the points previously reviewed, the relationships between radiation-induced iife-shortening and aging - if endeed the latter effect exists to justify such relationships - will not be taken up again for discussion in the rest of the present document. I. 58. PHYSICAL VARIABLES To establish a relationship between the degree of life-shortening and the characteristics of the acute or chronic exposure to radiation is important for the need to fix criteria and levels for human exposure. It may also be useful for theoretical reasons, in order to validate indirectly hypotheses and modeis on the nature of aging and on the similarity between natural and radiation-induced senescence. Experiments on single acutely-delivered doses are but one and the simplest out of all possible models: single-dose irradiations are not interesting in practice, but represent the most efficient treatment, since under these conditions the action of any repair system is minimal. On the opposite side, there are experimental treatments like the duration-oflife exposure which may more closely resemble the situations of interest in practice: they yield, dose for dose, less effect than the acute exposures. Between these two extremes there is a whole range of treatments where any given amount of effect can be obtained by infinite combinations of many interrelated variables. They are: the number and size of the dose fractions, the radiation-free time interval between fractions, the time over which a given radiation treatment extends, the total accumulated dose, the instantaneous dose-rate, etc. All these variables interact for any given radiation treat- ment to produce the final effect on survival and it is in practice extremely difficult to design experiments allowing their separate analysis. It should also be added that each experimental system has its own biological, physiological and pathological characteristics (to be examined under "Biological variables", see chapter II) and that an end-point such as life-shortening may be the result of an infinite number and type of underlying biological effects. 59. Having thus recalled the complexity of the problem at hand, the follow- ing irradiation conditions will in turn be considered; single acutely-deli-