hy 181. Conclusions On the whole, the modifications of life-shortening induced by changing the instantaneous dose-rate are rather variable. The conditions examined co- ver acute, fractionated and chronic irradiation, as well as various types and energies of the radiations. For single acute doses of low-LET beams, changing the dose-rate from 0.4 to 40 rad/min did not significantly alter the effect [S21]. When the intensity varied between about 80 and about 150 000 rad/min [L12] the effectiveness of the treatments differed as a maximum by a factor of 1.6. Thus, acute treatments show little dependence on the instantaneous dose-rate down to O.4 rad/min. At lower dose-rates, down to about 1 rad/day, it becomes difficult to resolve changes due to the instantaneous dose-rate and those attributable to dose protraction over a time which allows adaptation of the animal to the treatment. Under conditions implying irradiations for weeks or months, the efficiency of the treatment with respect to acute doses may drop by a factor of 10 or even of 25 for extremely low dose-rates and long irradiation times with accumulated doses involving less than 50 per cent survival of the irradiated animals [U7]. With such extremely protracted irradiations the form of the dose-effect relationship may change with respect to the very acute exposures with resulting lower effectiveness at lower intensities, but with various reduction factors for dif- ferent total doses [UT, U8]. 182, Examples have been reported where a relatively high loss of effective- ness was seen at doses of the order of 50 rad or less, while proportionately lower reduction factors would be applicable at higher doses. Modifications of the form of the dose-effect relationships are not surprising, since changes of the damage, repair and repopulation at the various doses would superimpose to irradiation. All these variables would be expected to change profoundly as a function of time and dose and therefore the situation operating under a given set of irradiation conditions would hardly be comparable to that of acute irradiation and could not be interpreted according to the same principles go- verning the response to an acute insult. 183. In the case of neutrons, changing the instantaneous dose-rate between 8 and 0.2 rad/min does not result in an appreciable loss of effectiveness [V1]. The data available at lower dose-rates (25 to 0.004 rad/day) given in protracted exposures [U7, U8] may be interpreted to show that the reduction of effect at the low intensities is modest, probably lower than a factor of 1.5. There may be question as to the significance of changes in the form of the curve