A cooling test was used to ascertain how long it takes for cold acclimatization to develop and how long it persists. A rat was taped to a board by its 4 legs. A thermocouple was placed in the colon and the temp. changes there were registered on a recording potentiometer from many individuals simultaneously. Unacclimatized and acclimatized rats were then placed at 5[degree] C until each cooled to 18[degree] C colonic temp.; their rates of colonic cooling were compared. Individuals living in cages in air of 5[degree]C for 2 days showed the same rate of cooling as those not previously exposed. After 4-10 days in the cold, cooling was 35% slower. This slower cooling rate persisted during 2 weeks of no exposure; thereafter it disappeared. Acclimatization was also conferred by virtue of the cooling test alone in 3 successive exposures on alternate days. Such acclimatization did not differ from that of rats exposed to cold continuously for 5 days. Rats which had lived in 5[degree] C air for 2 or more days, when taken for a few min. to 39,000 ft. simulated altitude lost their righting reflexes significantly more quickly than controls. Loss of righting reflexes occurred more quickly with increasing time spent in the cold, so that this test also may be used as a criterion of acclimatization to cold. Hence, exposure to cold made the rats less able to withstand altitude.