1. The fat content of the skin and of the skin-free carcass was measured in young, growing hairless mice about 4-8 weeks old kept at either 22 degrees (cool environment, i.e. below their critical temperature) or at 33 degrees (warm environment; i.e. within their thermoneutral range). The food intake of groups of the mice reared at each temperature was restricted to between 77 and 89% of that of a litter-mate fed ab lib. 2. In all the mice, whether fed ad lib. or on a restricted intake, those reared in the warm environment contained about 1-5 times as much fat as those reared in the cool environment. At both temperatures and feeding levels, approximately 25% of the total body fat was present in the skin. 3. Each mouse reared at 33 degrees on an ad lib. regimen reached about the same plateau weight in the same period as its litter-mate reared at 22 degrees. However its food intake during this period of growth was only about half that of the mouse reared at the lower temperature, so its food conversion ratio (dry food intake:body-weight gain) was twice as efficient. 4. It is concluded that neither the growth rate nor the distribution of fat within the body of the growing mouse can be influenced by rearing the animals at these different environmental temperatures. Rearing at the higher temperature, however, both decreases food intake and also promotes deposition of more fat within the body.