1. Rats 10 weeks old were fed for 9 weeks either on a stock diet containing 17% protein, or on a low-protein diet prepared from the stock diet with added glucose, minerals and vitamins. Half the animals on each diet were kept at room temperature (21°) and half in a cold environment (5°).
2. The calorie intake of the animals kept at 5° on both diets was 60–70% higher than that of the corresponding group at 21°. The animals on the stock diet and kept at 5° gained weight but not so much as those on the same diet at 21°. The animals kept on the low-protein diet at 21° lost weight, while those on the same diet at 5° lost only a little weight initially and none thereafter.
3. On both types of diet the liver, kidneys and gastro-intestinal tract weighed more per 100 g body-weight in animals kept in the colder of the two environments; the small intestine was conspicuous in this respect.
4. The weight of the fur was greater, and the weight of the skin less per 100 g body-weight at 5° than at 21°.
5. The animals on the stock diet at 21° had most fat in their bodies, both in absolute terms and per 100 g body-weight. There were no significant differences between the other three groups.
6. The skin of the animals kept at 5° had a significantly higher collagen to N ratio than the skin of those having the same diet at 21°.