also a protraction factor which takes up different values (from 2 to 5) as the exposure rate decreases from 40 - 50 R/day to 12 or less R/day. 178. Spalding et al. [S28] exposed 13 groups of adult female RF mice to 6056 gamma rays at 2.76 rad/hour until a predetermined dose (from 457 to 3.008 rad) had been accumulated. As doses increased, there was no accompanying increase of life-shortening because any dose had about the same effect, amounting to an average of 83 days reduction of life. Assuming a linear relationship with dose in the range 0-47 rad, the effect would be about 0.18 days/rad, a value which is on the low side of those usually found for low-LET radiation (see Table 1). 179. The contribution of Russ and Scott [R1] on the rat should only be men- tioned for historical reasons. Boche [B11] refers to some experiments in which rabbits received one year of treatment at daily exposures of 0.1, 0.5, 1.0 and 10.0 R. Life-shortening was shown in various ways. Similar treat- ments were also given to a few dogs and when the experiments were conducted with irradiation for only a fraction of the life-time at dosages below 1 R/day no effect on survival was noted. 180. This was also true for monkeys. The paper by Fritz et al. [F7] on the beagle dog is an attempt to cla- rify time-dose relationships after terminated chronic exposure to 606 gamma rays. Four exposure rates (5, 10, 17, 35 R/day) were used and the dogs (354 in total) were removed and allowed to die of natural death after total exposures of 600, 1400, 2000 and 4000 R. The experiment is still in progress but the provisional data are sufficient for a few tentative conclusions, as follows. The LD. 9 increased from 344 R (258 rad) delivered at 15 R/min to about 4000 R (about 3000 rad) at 10 R/day. Over this range of exposures the leading cause of death was haemopoietic damage. At 5 R/day or lower no definite LD. could be determined: the haemopoietic function continued at a nearly normal rate and survival was prolonged. The large number of malignancies other than leukaemia observed among the few animals dead up to the time of the report suggested that in irradiated animals tumours of the soft tissues would be significantly inereased with respect to controls. It is yet too early to see whether a non- specific component in the life-span-shortening might become apparent at the lowest dose-rates.