of fast neutrons. The median life-span of the controls was 828 days; of the 100 rad-exposed animals, 730 days (12 per cent reduction); of the survivors in the lethal dose range (120 - 160 rad), 698 days (16 per cent reduction). Both values of the median life-span were significantly smaller than control survival. In a related study 'K9? young adult male rats (94 to 110 days of age) were treated with 215 - 230 rad from the same neutron source. Although this dose was sublethal to the animals at 30 days, the median life-span was reduced by about 22 per cent. It was concluded that a dose range producing some acute mortality in the guinea-pig is less effective on life-span than a dose which in the rat is sublethal. The reduction in median life-span/rad is, however, comparable for the two species. 4. 85. Chinese hamster There are a few data obtained by Kohn and Guttman [K11] on the chinese hamster. Although this animal is more resistant to the acute effects of ir- radiation than other rodents under similar conditions, the late effects tend to be more severe, at least judging from the life-span-shortening. 550 rad of x ray whole-body In fact, exposure caused a loss of 32 weeks (corresponding to about 30 per cent) of the life-span remaining at the age of 230 days. At higher doses, for each increment of 100 rad above 550 rad and up to 950 rad there is an additional approximately linear loss of life-span of 20 weeks up to a per cent life-span reduction of 93 per cent. 5 86. Dog A large experiment on the life-span of normal and irradiated female beagle dogs has been reported by Andersen and Rosenblatt [A2]. At 10 - 12 months of age the dogs were given single or fractionated 250 kVp x rays treatments to total doses of 100 or 300 R. life-span than controls. All irradiated beagles had a shorter For single-dose treatments the life span-shortening relative to controls amounted 9.5 per cent and 20.7 per cent in the 100 and 300 R groups, respectively. The average life-span-shortening/100 R amounted to 6.7 per cent. rates were calculated for the last 6 years of life Mortality and the Gompertz slopes were found to be similar for all control and treated groups, except that the irradiated dogs attained higher rates of mortality earlier in life than controls. Major causes of death were tumours and chro- nic diseases (nephrosclerosis, heart failure, pancreatitis) with no obvious