to assess the contribution of each specific cause to life-shortening, since there is no reason to presume that each of them may be equally accelerated by radiation. Serial sacrifice experiments could, in principle, provide such information, but they require considerable time and effort and are therefore not common in the literature [K1, C3, A3]. 11. Apart from the difficulties in defining and describing a complex effect like life-shortening, its quantification could also create problems by giving implicit support to one or another possible interpretation. Interesting ob- servations have been made by Mole in this respect He points out that [M2, M3]. it is not immaterial to think of the effect as a differential between lifespan of the control population and the life-span of the irradiated animals, to - tp: If it is postulated that survival after a given dose is a function of that dose, it is implicitly ignored that animals may die of some unrelated pathology, and there is no implication that the effect of radiation may persist up to the end of life. If, on the other hand, the postulate is that the differential life time is a function of the dose, then the implication is that the effect of radiation may be equivalent to natural aging. No problem arises when single acute exposures are given at young ages or in duration-of-life experiments, because under these conditions the two formulations are identical. But when experiments to various to are involved, that is, when groups of animals are started on a course of irradiation at variable ages, one could eome to quite different conclusions from the same experimental data simply by following one or the other approach. 12. By definition, life-shortening is an effect that must be estimated stati- stically by comparing irradiated and non-irradiated animal populations. There are different ways of describing and expressing quantitatively the effect. They are the mean or median life-span, the percent cumulative mortality or the age-specific mortality rate. All may be regarded as compounded expressions over the whole population of animals of specific and non-specific causes, acting and interacting within each individual to decrease its fitness and to take him to death. 13. Life-shortening is expressed in days of life lost and since the time to death is a random variable, life-shortening may be represented by one of the following statistics: shortening of mean or median age at death; shortening