increased the slope of this function. Other techniques of analysis by the cumulative or the impulse lethality functions were also proposed as quasiempirical actuarial and kinematic approaches to the description of the effect. The 1955 paper by Sacher ‘S1] was a comparative review of radiation lethality in various mammalian species, particularly under conditions of chronic treatment for the entire duration of life. 3. It is of interest to note that, although the treatment of this subject had already proceeded quite far, there was a small coverage of it in the 1958 report of the Committee [U1], where the analysis of the biological endpoint was not very sophisticated. It has been established at the time that while the pathogenesis of early death was due to the failure of self-renewing systems in the body, the precocious extinction of an animal population as revealed by actuarial analysis, would be due to different mechanisms of action. However, there seemed to have been poor discrimination in the treatment of the two subjects. No attention was given, in particular, to the actuarial approach which had already been advocated by Sacher [S21 and Brues and Sacher B1}. 4, The review presented by Mole at the First International Congress of Radiation Research [M1! critically discussed on the basis of experimental evidence the idea that experimentally observed life-shortening might be equivalent to natural aging, a phenomenon about which too little was known to warrant unsupported generalizations. However, the notion that radiation could shorten life by similar physiological and pathological phenomena as natural aging gained momentum as a result of some observations on survivors of doses in the lethal range made by Henshaw [H1] and later by Alexander [A1]. The idea was first based on actuarial observations of an increase in mortality rate from all causes of death, with an apparent shift to earlier times of diseases characteristic of the late ages. Also, the occurrence in these sur- vivors of phenomena typical of the old age (graying of the fur, cataract, loss of reproductive capacity, etc.) tended to give support to the hypothesis. However, Upton in his 1957 [U3] and 1960 [U4] reviews warned against the establishment of close resemblances between certain effects of irradiation and aging, because not all age-dependent changes were affected similarly by radiation and the incidence and severity of the various diseases differed in control and irradiated animals. Under these conditions it would, of course,