Experimental studies of profound hypothermia were performed on dogs. Rapid cooling and rewarming were obtained by means of extracorporeal circulation combined with heat exchanger. Some physiologic and pathohistologic changes of several organs were followed at the various stages through the experiment. Following results were obtained. 1) There were found no marked differences in results between Kay-Anderson machine and bubble type oxygenator. 2) Changes of temperature of internal organs on cooling and rewarming were observed proportionately to vascularity of the given organ, most rapid in liver and most lagging in muscles. Changes of rectal temperature were not constant. 3) Pupils of dogs became dilated on cooling, maximum during circulatory cessation, and retuned to previous size on rewarming. 4) On histologic study of liver, appearance of vecuoles was observed at both stages of cooling and rewarming, which were proved to be neither fat nor glycogen by histochemical examination. They were considered merely watery vacuoles and reversible changes. Marked decreaae of liver glycogen was observed on both cooling and rewarming and its disappearanse was first observed at the vicinity of central vein. 5) Renal congestion and claudy swelling of epithelial cell of renal tubules, especially in proximal part, were prominent, although these changes were considered reversible. 6) Stagnation of blood was a main change found in the histologic study of heart, lung and adrenal gland. It was most prominent in liver, kidney and spleen, however, degeneration or necrosis of the tissue were not found at all. 7) Based on these experimental data, profound hypothermia is considered applicable to the clinical case and better results will be expected with the use of low molecular weight dextran.