lationships obtained for high dose-rate acute exposures (see Figures II and III) and those operating under duration-of-life conditions (see Figure X). However, the most striking finding in these series is the modification of the dose-effect relationships taking place at progressively variable dose rates. These modifi- cations, depending on the characteristics of the species and strains, are the variables determining the final outcome of any given course of irradiation. There are not enough data on any single species for analysing such a broad statement into any coherent model of action, particularly since published data have essentially been obtained on two mouse strains [M7, U7] and other series !S11, S21, L6] have contributed relatively less information. 190. These data, involving x~ and gamma-ray and neutron irradiations were analysed by Grahn and Sacher [G1] and their conclusions may be accepted provisionally. Apparently (see Figure XIII) the curve describing life-shortening as a function of total dose accumulated for doses of 1500 rad or less and/or for protraction periods of 50 days or less shows an abrupt slope transition t . tT ae tor I FF v to : ' + T q 1 oF I + qT $ qo ' SOR 25 300 7 20 > < oJ Oo ~ > 4 — = ur b oO 4 200 © 7 = w tO J uu LL “ - MOoxpdeR@® J JOO i I 4 | 3 - . neutrons, single dose neutrons, >30%of lifetine ~ gamma rays, SO regs/d, term . gemma rays, [Srads/d, term 4 gomma rays, single dose gamma rays, 25 recs/d,term camma rays, S rocs/d, term gemma rays, lifetime, theoretical - 1 J... 1. } 4 ad f l 5 ACCUMULATED DOSE, KILORAD Figure XIII. Itfe-shortening as a funetion of mean aecumlated dose tn RF female mice given vartous patterns of exposure (single, chronic terminated, duration-of-life) to gamma rays and neutrons. Data from Upton et al. |UT! as plotted by Grahn and Sacher |G1]