The dosimetry was such that the animals in the room at 25°C received from 38 to 96 mR/24 hour-day and the exposed ones 895-931 mR/8 hour-day. at 5°C the respective dose rates were: control 42-149; In the room irradiated 897-966. Oxygen consumption, food consumption, body weight and metabolism were checked routinely. The irradiated animals at both 5°C and 25°C lived over 20 per cent longer than their respective non-irradiated controls, the half-lives observed being: at 5°C, control 240 days, irradiated 305 days; days, irradiated 600 days. at 25°C, control 460 No explanation was offered for this observation, except the suggestion that a mild injury might result in apparently beneficial effect by stimulation of cell and tissue repair and repopulation processes. 292. Trujillo et al. [T1] reported that RF/Un female mice showed a linear de- crease with increasing age in their ability to withstand a standard cold stress (6°c to 7°C for 14 days). Mice exposed to protracted 6006 gamma-ray exposure at 50 rad/day and then allowed to recover for 90 days showed a similar linear decrease with increasing radiation exposure in their ability to withstand the same cold stress. This radiation-induced effect was considered similar to life-shortening by natural aging and was equivalent to 0.093 day/rad. 293. In a rather more elaborate set-up Carlson and Jackson [C22] studied the interaction among radiation and high temperature. into 8 groups of 22 rats each, individually caged. The animals were divided Four of them were kept at 28°C and exposed to 0.29, 0.64, 2.60 and 4.18 R/day during the year that irradiation lasted from 4 to 16 months of age. The other four groups were kept at 35°C for the same length of time and exposed to 0.28, 0.60, 2.57 and 3.96 R/day. The age at which 50 per cent of the irradiated animals died increased with increasing dose at all dose levels tested, with good statistical significance of the data, except for two points. It was suggested that ionizing ra- diation may have interacted with the environment in increasing longevity by stimulation of the repair processes. These experiments [C21, C22] are note- worthy, not only because they show an interaction between radiation and ambient temperature, but also for the finding that radiation in the region of 4 R/day or lower has vival. (at all temperatures tested) increased, rather than decreased, sur- As to the first point, the observation has so far remained without con- firmation; concerning the second one, it should be mentioned that Bustad et al. [B17] working on individually caged mice within the same exposure range but at normal ambient temperature could not confirm the data of Carlson. The problem therefore remains unsolved, since the different species or environmental con- ditions might have been responsible for the negative observations of Bustad.