1. For laying fowls fed on a conventional diet increasing the environmental temperature from 21 to 27 °C at 333 d of age resulted in a depressed rate of egg output (333 to 500 d). 2. Egg output for birds kept at 27 °C from 333 d could not be increased to the maximum rate of output of birds at 21 °C for this period by feeding a diet of equal energy content containing increased proportions of protein-rich ingredients. 3. The difference in both food intake and egg output between hens maintained at 21 and those maintained at 27 °C appeared to decrease when the higher temperature was introduced at 416 d of age.