The hamsters to be resuscitated had all been without respiration or circulation for many minutes or hours and appeared dead. Those which had frozen progressively for 50 min or more at -5 degrees C were sufficiently rigid to be supported by the neck and tail without collapsing. The whole body was rewarmed by means of diathermy, by gentle illumination at room temperature, or by immersion in warm water. Artificial respiration was usually administered throughout rewarming or as soon as partially frozen animals were sufficiently thawed to permit insufflation of air. The rate of recovery, the proportion of animals completely reanimated and the subsequent mortality depended upon the method of resuscitation. A diathermy technique was evolved which proved more effective than other methods. The previous duration of suspended animation and of progressive freezing also influenced the course of revival and the proportion of animals which recovered completely and survived. Hamsters which had supercooled to temperatures as low as -5.5 degrees C revived fully and lived for many weeks or months thereafter. Few animals which had crystallized spontaneously after supercooling to temperatures below -3.5 degrees C recovered fully and none survived more than a few days thereafter. A high proportion of hamsters which had frozen progressively for 50 min or less recovered completely and survived indefinitely. Seventeen of the twenty hamsters frozen for between 50 and 70 min recovered and, of these, eight survived long periods. Animals frozen for longer than 70 min seldom revived fully, although spontaneous breathing was resumed in those frozen for 90 to 159 min and heart beats in those frozen for 160 to 170 min. Autopsies showed that the chief cause of death was haemorrhage in the stomach, intestines or lungs. Frostbite and lesions of the eyes were rare, and no changes in behaviour were noticed in fifty-nine animals which were studied for 100 to 450 days after resuscitation.