This experiment was carried out to study the responses of cellular component of blood and bone marrow to cold and also the changes of coagulation during cooling. Forty-two mongrel dogs were subjected to hypothermia by ice-water surface cooling technique. Lowest body temperature ranged from 21-23 degree. Dogs were divided into 3 groups,Group I, 12 dogs: pentothal anesthesia for 3 hours, Group II, 20 dogs;hypothermic group and Group III,10 dogs;postsplenectomy hypothermic group. Results were summarized as follows: 1. Hemoglobin, hematocrit and red blood cell count significantly increased when animals were cooled, and increase was noted in similar magnitude among the animals of Group I. 2. White blood cell count extremely decreased after cooling and effect of splenectomy on white blood cell count was not apparent. No significant changes were seen among Group I. 3. Differential count of white blood cell when cooled showed relative increase of polymorphonuclear neutrophil and decrease of lymphocyte. 4. There was marked decrease of platelets when body temperature reached to 21-23degree and essentially. no changes was noted in Group I. 5. Clotting time, bleeding time, plasma prothrombin time, recalcification time, and fibrinolysis showed no significant changes when dogs were cooled. Clot retration and prothrombin consumption during hypothermia appeared to be poor. In Group II, bleeding time decreased after splenctomy and when body temperature was lowered, plasma prothrombin time, clot retraction, and prothrombin consumption decreased. Decreased bleeding time and poor clot retraction were noted in Group I. 6. It was found that megacaryocyte count decreased even though platelet count of peripheral blood markedly diminsished when animals were cooled. There was some tendency of erythroid hyperplasia noted during hypothermia. [KTCS 1969;1:105-114]