288. Among the many hypotheses considered it appears that, at least under some conditions, the effect of age might be attributed to a decreased susceptibility of certain pathological conditions. Another possibility is that radiation in old age may have some therapeutic effect on some (presumably neoplastic) con- ditions already under development at irradiation. Caution should be used in the analysis of data where animals are started on irradiation courses at variable initial ages since different analytical appraoches to the data, in additio to implying different hypotheses of action, may lead to variable conclusions as to effect of age on the animals radiosensitivity. 289, In two large sets of experiments [S19, S20] a number of texts (LD. 4 739° survival time after daily exposures to 100 R, recovery rate after split doses) were performed on RF or BDF1 mice at various times following a conditioning dose of radiation. The experiments showed that each test measured the damaged inflicted to different cellular systems and that each system aged according to a different rate. The data showed also that normal animals exhibited a sharp decline of their radiation resistance in old age. Previous irradiation induced an increased susceptibility and an earlier onset of the decline, which was interpreted as the expression of the earlier onset of morbidity in the irradiated animals. Itt. A. 290. MODIFYING EFFECTS PHYSICAL TREATMENTS Among the treatments that modify the life-shortening response to irradi- ation those of a physical, chemical or pharmacological, and biological nature will in turn be reviewed. It should be realized that information available on these subjects is very heterogeneous and not suitable for generalized conclusions. Among the physical treatments, those referring to irradiation given in combination with low- or high-temperature treatments should be included. 291. Carlson et al. [C21] exposed male Sprague-Dawley rats to 20°C and 5°C to 609, gamma-irradiation for 8 hours/day during one year. The animals were caged individually and a parallel control group was run at each temperature.