Some mice were cooled to a deep body temperature of 15°C, others in ice for an additional 20 minutes to 1°C, still others in ice for 45 minutes to 1°C. The first group showed no impairment on a test for CNS function; the second produced equivocal results; in the third, transitory impairment occurred in skilled mice and in others, slowness in acquiring skill. This third group also showed decrease in weight gain and a mortality rate, especially among the heavier animals. It is inferred that duration as well as intensity of hypothermia is significant, and that a critical point is reached after 20 minutes in ice. Histology revealed no CNS damage. Submitted on October 27, 1958