The changes of the contents and activities of antithrombin III (AT-III) in plasma of rats following frostbite of both hind feet were investigated by means of rocket immunoelectrophoresis and single immunodiffusion. The results showed that the contents and the activities of AT-III were decreased in plasma of rats after frostbite and these changes were closely related to the degrees of cold injury. It is suggested that as a result of depressed anticoagulability, the blood coagulability could be increased after freezing, leading to the dysfunction of circulation and necrosis of local frostbitten tissue ultimately.