1. 103. Mouse Henshaw [H1]| carried out two small experiments with C3H mice with 200 kV x rays (0, 5, 10, 20 and 25 R/daY, 5 times per week). He showed a decrease of the average survival time down to 58 per cent of control at the highest exposure rate and an increase of the death rate. In a later re-examination of these data Boche [B11] showed that the excess death rate over the control death rate divided by the exposure rate was a constant within the range of exposures considered and could be taken as a characteristic of the mouse sensitivity to chronic x-radiation. 104. More complete experiments by Henshaw [H6] and Henshaw, Riley and Staple- ton [H7] were reported on four strains of mice (CF1, ABC, C58 and A), selected for special characteristics, like, for example, the high spontaneous incidence of leukaemia and lung tumours. Daily whole-body exposures to fast neutrons (0.115 to 13.0 n) and of gamma rays (1 to 17.2 R) were administered 6 times weekly at high instantaneous dose rates. Most of the animals died either in a state of atrophy or with mediastinal lymphomatosis. Doses and LET dependencies were noted, as well as changes in sensitivity between strains and special pat-— terns of response of the various diseases. In those days the main interest was in establishing the existence of a threshold dose: in this respect that of neutrons was set below 0.115 n/day and below about 1 R/day for the gamma rays. It was also noted that life-span was a sensitive indicator of damage since about ten times more radiation was required to produce a threshold pathological change than to shorten appreciably the life-span. 105. Daily doses of 0.014 n of fast neutrons (equivalent to 0.1 R/day of x rays on the basis of the relative effect of single exposure) were shown to have no definite effects on mice treated for 87 weeks. Survival time was de- finitely reduced at exposures of 0.14 n/day for a total of 60 n, while doses of neutrons ten times higher produced over 50 per cent death of the animals by about the 29th week at a total dose of approximately 200 n [E2}. 106. Boche [B11] studied various species of animals. Wistar rats were treated with 250 or 1000 kVp x rays at exposure levels of 0.1 — 10.0 R/day. Rabbits received 1 year of irradiation and beagle dogs or monkeys were treated for 2 years at similar exposure levels. The life-span~shortening effect was shown in various ways but the different energy of radiation or