The effect of hiberation on the radioresistance of the adult albino rat was studied. Hibernation increases the resistance of the rat submitted to a LD/ sub 100/ irradiation by gamma rays and thermal neutrons. The rats were hibernated by the Calm and Baclesse raethod and the rectal temperature was lowered io 20 deg C. They were then irradiated in a reactor with 1000 r. It was shown that the increase of radioresistance was caused by the hibernation and not by the drugs injected. The protective effect of cysteine is also increased by hibernation. However, an injection of pentothal plus hibernation and cysteine or pentothal and cysteine alone give a toxic complex at doses which are, if the products are individually injected, not toxic at all. (auth)