1. It is clearly shown that the diet and general body condition of sheep markedly influenced their heat-regulating mechanism.
2. The maintenance of the body temperature of thin sheep in a cold environment was found to be closely associated with the calorific value of the diet at the time of exposure.
3. Moderately conditioned sheep maintained their body temperature when exposed to cold for a period of 10 days after being placed on an inadequate ration.
4. The control of temperature in a hot environment was found to depend largely on general body condition.
5. When the sheep were exposed to heat the following observations were made:
(a) Thin sheep on a poor diet showed a suppressed panting reflex and an excessive rise in body temperature.
(b) Thin sheep on a good diet showed a normal panting reflex but also an excessive rise in rectal temperature. This anomaly cannot as yet be fully explained. The ability to control the body temperature returned when the general body condition improved.
6. Contrary to expectation, continued poor feeding of sheep in conjunction with repeated exposure to cold failed to cause any clinical disturbance either in digestion or in the normal appetite of such animals. The effect of this treatment on the heat regulating system of the body, however, was both clear and pronounced.