1. Eight calves of each of two breed types, Friesian (Fr) and Brahman × Friesian (FB), were fed ad libitum on one of two diets; the digestibility of the dry matter of the diets was 72% and 63%. Eight calves were kept at 34°C and eight at 17°C. In a second experiment six of the calves previously exposed to 34°C were subjected to daily exposures to a radiant heat load at 32°C air temperature, while fed on the diet of lower digestibility.

2. Rectal temperatures and respiratory rates were higher, and food intakes and growth rates were lower, at 34°C than at 17°C. However there was only a small difference in growth rate between the two temperatures after the effect of temperature on food intake had been removed by regression.

3. At 34°C FB calves had lower rectal temperatures and respiratory rates than Fr calves, and when fed on the diet of higher digestibility FB calves ate more and grew faster than Fr calves at both temperatures; however there were only small corresponding differences between the breed types for calves fed on the diet of lower digestibility at both temperatures.

4. At 34°C FB calves had lighter (weight) hair coats than Fr calves and FB calves had lighter coats at 34°C than at 17°C; the latter difference was not apparent for Fr calves. Nevertheless, neither rectal temperature nor respiratory rate was significantly affected for either breed by removal of the hair coat at 34°C.

5. In the second experiment the rectal temperatures of FB calves increased by smaller amounts during exposure to radiant heat than was the case for Fr calves; FB calves had faster sweating rates than Fr calves during these exposures.