within such a small rnage of variation. There has been experience in changing the instantaneous dose-rate (from 13 to 1 rad/min) in the course of brief repeated exposures at daily or weekly intervals. For daily exposures, a dose- rate dependence of 606, gamma radiation amounting to a factor of 2 has been found, but not for fast neutrons. Weekly fractions with higher doses and with- in a higher range of dose-rates (1 to 35 rad/min) did, however, show some re- duction of effectiveness even in the case of neutrons [V3, V4]. 18h. Thus, the dependence of life-shortening on instantaneous dose-rate is modest for low-LET radiation and doubtful for neutrons, for treatments lasting a few hours to a few days. Only when extremely low dose-rates and correspon- dingly long irradiation times are involved, x- and gamma-rays (but not neutrons) show consistent reductions of effectiveness, of the order of a factor of 10 or 20. Under these conditions, however, there is no way to discriminate between the effect of lowering the dose-rate and that of protracting the treatment time, which implies re-equilibration of the organism's adaptation system. 185. Experiments performed by splitting a given dose into two or more frac- tions separated by a few hours to a few weeks have repeatedly been performed. won The simplest instance is one where two dose fractions, equally or unequally - divided, are administered within fractionation times of the order of a few hours to one day. Two experiments with such a scheme [G1, S27] have yielded little increase in survival by the split dose: life-shortening was within 5 - 8 per cent of the single-dose survival time in the first case and the results were essentially negative in the other. 186. Experiments using two dose fractions in the range of 100 rad or more total dose for fractionation times of 1 day or longer are more numerous [U11, C18, K6, A2]. Fractionation intervals ranged from 2 days to 8 weeks. the dose usually produced less life-shortening. Splitting In two cases [K6, U11] a spa- ring effect of the order of 3 - 20 per cent was reported. In the other two eases, no effect of dose fractionation [C18] or a negative effect, that is, an increased life-shortening, was observed [A2]. When fractionation intervals of progressively longer duration were tested with a given scheme, a tendency to a longer life-span with increasing interval between doses has sometimes been found, but the variations observed even for very long fractionation times are too small to make these observations clearly significant [G1, A2, S27].