1. C57xRIII F1 hybrid mice reared at 32° C. D.B., 29° C. W.B., from 3 to 8 weeks of age, survive longer at 41·7° C. D.B., 29·4° C. W.B., than litter-mates reared at 20° C. D.B., 16° C. W.B.

2. The transference of animals from the hot environment to the cooler one 48 hr. before they are exposed to the lethal temperature has little or no effect on their heat tolerance; but transference in the opposite direction greatly increases survival time.

3. Mice whose tails have been amputated 5 weeks before they are exposed to the lethal temperature have a lower heat tolerance than normal animals.

4. The total loss in weight of an animal exposed to the lethal temperature is independent of the environmental temperature at which it has been reared; but heat-acclimatized animals lose weight less rapidly than control ones.

5. It is concluded that at least some of the changes, both physiological and morphological, which occur when mice are reared at high temperatures, are in their over-all effect adaptive.

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