1. High temperature decreased food intake, growth rate, energy retention as protein and water retention but had no effect on energy retention as fat and little effect on total energy retention. 2. High temperature did not affect the proportion of apparent metabolisable energy retained as protein or the efficiency of energy retention from apparent metabolisable energy. However, it increased the proportion of apparent metabolisable energy retained as fat. The latter effect was attributable to the reduced energy requirement for thermoregulation. 3. Offering the birds a choice between a "high-protein" and a "high-energy" diet did not improve growth when the animals were exposed to high temperature (30°C). Moreover, choice-feeding led to decreased food intake and, in turn, growth rate in birds kept at moderate temperature (20°C) when compared with those given a complete compound diet. 4. Compared with birds kept at 20°C and fed ad libitum, there was a lower concentration of plasma triiodothyronine in birds kept at 20°C but pair-fed the same amount of food as birds eating ad libitum at 30°C. Triiodothyronine concentration was shown to be affected directly by food intake as well as by ambient temperature. 5. High temperature reduced the quantity of digesta in the gut. It was suggested that this may be due to a reduced ability of the gut to process food. 6. Dietary modifications such as finely grinding and wetting the food led to increased food consumption and growth rate at high temperature. 7. Increasing the sodium chloride content of a finely ground and wetted diet improved metabolisability. 8. Increasing the protein content of a finely ground and wetted diet increased body weight gain but caused decreases in total energy retention, protein retention efficiency and metabolisability. 9. It is usually assumed that reduced food intake at high temperature results firstly from a decrease in energy requirements for thermoregulation and secondly from the need to minimise heat stress by reducing the heat increment of feeding. However, the results of this project suggest that the domestic fowl reduces its food intake at high temperature not to reduce or limit heat production, but because the gut is limited in the amount of food it can process. If the gut is "assisted" by providing wetted or finely ground food, food intake, growth rate and heat production all increase.