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Two groups of male albino rats of 26 days old, were kept at 5°C and 28°C respectively. Their growth was studied by weekly gross measurements and biweekly radiographic investigations for 16 weeks. Compared with the controls at 28"C, the rats kept at 5°C showed the following effects of cold in their growth: (1) Smaller measurements at all ages; (2) Slower growth rates in the first two to four weeks; (3) Relatively shorter tail and shorter feet but body weight was the same as the controls of the same body lengths irrespective of age; (4) Shorter but relatively thicker bones; (5) A disto-proximal stunting gradient was observed in the tail and limb bones, showing more severe effect in the distal segments and decreasing effect in the proximal segments; (6) Retardation of skeletal maturational status in the naked appendages (tail and foot); (7) Decreased number (1.4) of ossified caudal vertebrae.The above phenomena were discussed as results of possible reduction of blood flow to the skeletal tissue on cold exposure.It has been well known that the growth rate of rats and mice varies with changes of the environmental temperature. Many previous works were planned to compare the growth of these small mammals reared under two contrasting experimental temperatures. As far as body weight and body length are concerned, the results of the previous works have been inconsistent. They can be classified into the following two groups. 1. Ogle ('34), Ashoub ('58) and Harrison ('63) kept growing mice at a higher temperature ranging from 31" to 33°C and in a lower temperature of 10°C to 21°C. The mice reared in the lower temperatures grew heavier and longer than those reared in higher temperatures. 2. Sumner ('15), Hgroux and Gridgeman ('58), Chevillard, Portet, and Cadot ('63) and Barnett ('65) kept their young animals in a higher temperature of 21" to 30°C and in a lower temperature of 3" to 7°C; the cold animals became lighter and smaller. All reports agree, however, in the increased length of the tail in rats from the warmer environment and in decreased tail length in rats from the colder environments, irrespective of temperature ranges.The growth in body length and body weight of rats and mice should be greatest under the optimum or the most favourable temperature ranges, and likewise retarded or interfered with under extreme and unfavourable conditions. either too warm A M , J. ANAT., 124: 23%250.

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