A high proportion of hamsters which had frozen progressively for 40 to 60 min in baths at -5 degrees C have been resuscitated (see part II). Transverse sections through other animals frozen for similar periods showed that the skin and body wall were frozen hard and that there was ice on the outer surface of the abdominal viscera and brain and within the substance of these organs. Calorimetric determinations showed that between 15 and 45% of the body water had been converted to ice after animals had frozen for 40 to 60 min. Calculations from the lowest temperatures reached showed that 53 to 63% of the water in the brain and 57 to 90% of the water in the subcutaneous tissue had been frozen within 1 h. Approximate estimates, based on the final colonic temperature reached during freezing, were made of the proportion of body water converted to ice in hamsters subsequently resuscitated. They showed that every animal in which 15% or less of the body water had been frozen recovered completely. Two-thirds of those in which 15 to 40%, and one-third of those in which 40 to 50% of the water had been frozen were fully resuscitated and survived long periods. Hamsters in which 55 to 70% of the water had been frozen subsequently recovered heart beats and breathing but not consciousness, whereas when 76% of the body water had been in the form of ice resumption of heart beat was the only sign of life. The rate of heat loss from hamsters was controlled by the rate of heat transfer through their skins and increased greatly after the integument had frozen. The biological and physical factors which may limit the viability of animals during chilling, freezing and thawing, and the possibility of storing whole animals for long periods in suspended animation in the frozen state are discussed.