ministration or the duration of exposure (and therefore the cumulated total The common feature is that in all doses) are the variables under examination. eases exposures were terminated before a substantial part of the animals came to death, thus allowing estimates of the relevant factors under conditions in which the "wasted radiation" component (see paragraphs 98-101) would not be effective. 174. The early contributions of Evans [E2! and Lorenz et al. only be cited for completeness. The paper of Mole and Thomas [L6] should [M7] is a syste- matic investigation of life-shortening in CBA mice by changing the duration of exposure to daily irradiation of 6046 gamma rays or of fast neutrons (mean energy 0.7 MeV). In the case of gamma irradiation, weekly exposures of 350, 210, 110 and 16 R/week for progressively longer times (4 to 30 weeks) and for the duration of life were tested. For neutrons, 16 or 2 rad/week for 5 to 60 weeks or for the duration of life were the conditions tested. For both radia- tions, as the duration of exposure to a given daily dose (and therefore the total dose) increased, the mean survival time decreased: however, beyond a given point a further increase of exposure time and total dose produced no more effect. The lower the daily dose, the more survival time became indepen- dent of the total dose or of exposure time. The minimum dose or exposure time required to produce a maximum life-shortening effect could only be approximated. The shape of the cumulative mortality curves depended systematically on the particular level of daily dose and on the duration of the exposure, except possibly at the lowest daily doses. A summary of these data is given in Figure XII. 175. Mole's paper [M7] contains, in addition to valuable experimental obser- vations, a number of interesting points for discussion. Some pertain to the concepts of reparable and irreparable injury (see paragraphs 18-34). Other points concern the problem of controlling and assessing biological variability in long-term experiments and here data are given for a discussion of the secular changes of the life-span of the controls. Another observation relates to the presence of discontinuities in the response to chronic irradiation generated by biologically different modes of death. Data on the effect of acute versus chronic irradiation were also reported in the mouse by Curtis, Tilley and Crowley [C19] and are discussed under paragraph 199.