Effects of pharmacological hibernation, induced hypothermia and administration of Cortisone & ACTH to the vital organ cells, were investigated histologically and the following results were obtained. 1) There were the most deleterious changes in the tissue cells under hypothermia without any anesthetics but the least under hypothermia with Phenothiazines. 2) The liver, kidney and lung were chiefly damaged, which were mainly circulatory, by body cooling. However, those changes of the organs were reduced by administration of Phenothiazine-derivatives, Cortisone or ACTH. 3) Histological changes were not increased by operative measure under hypothermia and pharmacological hibernation. 4) Glycogen in the livers, staining by PAS, decreased under hypothrmia but the decreasing was minimamized by administration of Phenothiazines, Cortisone and ACTH. 5) Decrease of nucleic acid in liver cells, by staining of nucleic acid, was noticed under hypothermia. 6) Various parenchymatous changes of the liver and kindney under hypothermia were thought to be caused by circulatory anoxia. 7) It was difficult to determine the limit of body cooling only by histological findings, but clarification of the cause of late death after hypothermia by histclogical findings was important. 8) Tolerance of rats to cold were increased by administration of Cortisone or ACTH, therefore these administration were very effective measures in order to save the lives under hypothermia.