C. 151. DOSE-RATEs« DOSE FRACTIONATION, CHRONIC TERMINATED EXPOSURES Other experiments are available where radiations of different types were given at various instantaneous dose-rates in chronic terminated or in fractionated exposures to animals of various species. The experiments were made sometimes to examine the effect of a given regime of chronic terminated exposure; in other instances, to study changes in the instantaneous dose-rate; in still other cases in order to compare the effects of splitting a single dose into fractions separated by various time intervals. In all these experiments the interplay of dose-time parameters is extremely variable and the final ef-~ fect may be expected to be intermediate between the two extremes of the single or of the duration-of-life exposure. It is difficult and somewhat arbitrary to separate all this work into various chapters: the subdivision to follow will consider separately the effect of dose-rate, of dose fractionation and of chronic terminated exposure, in an attempt to draw more systematic conclusions about the different radiobiological variables. 1. 152. Instantaneous dose-rate The papers where instantaneous dose-rate was examined as a separate va- riable in acute exposures or in the course of chronic terminated experiments are very few and all on mice. It is evident that, unless the dose-rates used are very high, the effect of the duration of the exposure cannot be separated from the effect itself of the instantaneous intensity of irradiation. 153. Vogel, Frigerio and Jordan [V1] reported that between 1 and 4.4 rad/min of fission neutrons the effect of life-span was independent of dose-rate, while there was dependence for gamma rays in the range of 1 to 13 rad/min, the efficiency of the treatment being lower at low dose-rate. were performed on CF1 The experiments female mice irradiated with 13 brief daily exposures. In another series the mice were irradiated with single neutron exposures at 8 rad/min or 0.2 rad/min (36-174 rad total dose). Survival did not differ within the above dose-rates. 154, Vogel and Jordan [V3, V4] irradiated the same mice with fission neu- trons and gamma rays (1 to 35 rad/min). of 60 Four weekly doses of 200 rad/week “Co gamma or 100 rad/week of neutrons were employed, delivered at variable