The effects of hibernation on survival after irradiation was studied in thirteen-lined ground squirrels (Citellus tridecemlineatus). The${\rm LD}_{50(90)}$ for a single dose of60 Co gamma radiation was 1024 rads for ground squirrels irradiated and maintained at 23°C, whereas the${\rm LD}_{50(90)}$ was 1127 rads for ground squirrels irradiated during hibernation and subsequently maintained in the cold room at 5°C for 3 weeks before they were returned to 23°C. Survival was higher in ground squirrels aroused from hibernation 1 day after an exposure to 1160 rads (79%) than in ground squirrels aroused from hibernation 2 weeks after exposure to 1160 rads (37%). The squirrels that were given 1160 rads at 23°C and maintained at 23°C had 9% survival. When ground squirrels were given two doses of 668 rads, 6 hours apart, survival was greatest (73%) in the hibernating group that was kept in the cold room 3 days before, during the interval between, and for 20 hours after the second dose of gamma rays. The next highest survival (46%) was observed in the hibernating group that was treated the same, but was aroused immediately after the second dose of gamma rays. The hibernating groups that were aroused between the two exposures to radiation had no greater survival (29%) than the euthermic controls (29%).

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