1. Rectal temperatures and respiratory rates have been determined in four calves during 5 hr. exposures to ten different hot environments in a climatic room.

2. Among the various parameters based on rectal temperature response, final rectal temperature proved the best for differentiating between the heat tolerance of individual animals. Under conditions of severe heat, where rectal temperature rose almost linearly with time of exposure, tolerance time proved equally suitable.

3. Since initial rectal temperature tended to parallel final rectal temperature, the increase in rectal temperature during exposure did not vary significantly between the animals. Increase in rectal temperature was therefore considered a less suitable measure of heat tolerance.

4. Both the lowest and the highest panting rate were associated with a low heat tolerance. From this, as well as from other observations and considerations, it was concluded that respiratory rate, either alone or in combination with rectal temperature, was an inadequate measure of heat tolerance.

5. A state of high heat tolerance tended to be associated with only a small loss of body weight during exposure to heat, but showed no relation to body weight itself.

6. The order of heat tolerance of the four animals was essentially the same in each of the ten hot environments investigated. It was inferred that within the range of temperatures employed in this study the degree of severity of the heat stress was not an important factor in the discrimination of heat tolerance.