electrons (15 MeV, 40 000 rad/min) they had a considerably reduced life-shortening effect, by comparison with other animals exposed in air. The protective ef- fect of hypoxia was influenced by the age at exposure in that a dose-reduction factor of about 3 due to the nitrogen breathing was observed in animals of 8 and 30 weeks of age; in mice of 1 day or 1 week of age the shape of the doselife-shortening relationship was changed from linear to curvilinear, giving rise to a larger protection factor at low doses and a very small one at high doses. 300. Hypoxic hypothermia was also tested by Hornsey [H12] in respect to the possible modification induced by this treatment on life-span. While hypothermia induced at the time or irradiation offered considerable protection to the haemopoietic system, whose failure is responsible for the early death of the animals, it did not protect to the same extent against long-term death. For the same dose administered to normal and to chilled animals the expectation of life was greated for the latter, but the nature of the data did not allow any precise estimate of the protection factor afforded by hypoxie hypothermia. Thus it appears that the protection by hypoxia already shown against the acute radiation effects extends also to the long-term effects, although perhaps not to the same degree, 2. 301. Chemical radioprotective drugs On the subject of chemical radioprotection Maisin et al. [M33] reported that mercaptoethylamine [MEA! (10 mg/rad, given 5 minutes prior to irradiation) was active in reducing the mortality rate during the first month post-irradiation but was incapable of modifying the late rate of mortality following irradiation of the head (1000 ~ 2000 R) or of the abdomen (900 - 1500 R). This drug was also without effect on the late mortality following irradiation of the abdomen and of the whole body with 600 R. In another series of experiments, MEA (425 mg/ kg/day) and 2-aminoethylthiosulfuric acid (1000 mg/kg/day) were administered in the drinking water to Swiss mice that were exposed for the duration-of-life to 606, gamma rays at dose rates from 1 to 5 R/hour. Mortality data were indis- tinguisable from those of controls drinking tap water and it was therefore concluded that neither of the drugs (which are active in the prevention of early mortality) had a protective action against chronic irradiation efffects at drug levels accepted by the mice /A8].