tion. The effect was dose-dependent in that it could be seen at the two highest but not at the low exposure. Spacing of the fractions at intervals of 3, 5, 7 or 14 days did not influence longevity. It was concluded that frac- tionation after exposure in the 1 to 6 fraction range led to 30-46 per cent of the sparing effect obtained by halving the total exposure in the 0 - 446 R exposure range. 171. In the Wistar rat Reincke et al. [R3] examined the effects on tumour frequency and life-span-shortening of two different fractionation regimes of whole-body x-irradiation (300 R in 3 exposures over 2 months or 10 R in 90 exposures at daily intervals) and compared these regimes between themselves and with control non-irradiated rats. No significant difference in the over- all incidence of tumours or on tumour types were observed. vival times were 20 and 25 months, respectively. The average sur- Tumour induction rates and death rates were not different between the two groups, but some differences became apparent when age-specific rates due to neoplastic and non-neoplastic causes were compared. In general, all radiation effects were more pronounced in the group receiving 300 R x 3. There was no complete summation of the effects of the single doses and recovery processes were more effective after small daily exposures than after greater radiation exposures given at longer intervals. 172. Various schedules of fractionation as well as single exposures to 250 kV xX rays were given to female beagle dogs by Andersen and Rosenblatt [A2}. For total exposures of 100 or 300 R, 4 equal fractions or 2 equal fractions given at 7, 14 and 28 days interval were administered. In general, differen- ces between subgroups receiving fractionated exposures were apparent only in groups totalling 300 R. In these animals life-shortening was increased when the total treatment time increased from 7 to 84 days. It was estimated that the decrease in life-shortening produced by 300 R would be reduced from 23 to 10 per cent as treatment time increased from 7 to 84 days. 3. 173. Chronic terminated exposures The papers discussed in the next few paragraphs (173 - 180) pertain to chronic exposures carried out with different radiations on various animals. The heterogeneity of this work does not detract from the quality of some of the con- tributions in which many important factors such as the protraction of dose ad-