Broiler chicks 4 and 4.5 weeks of age were subjected to the following temperature regimens for the experimental periods which ranged from 18 to 24 days in three experiments: cyclic cold temperature (1.7 to 12.8 C), cyclic hot temperature (23.9 to 35 C), cyclic normal temperature (12.8 to 23.9 C), and "changing" temperature. The birds of the "chaning" temperature regimen were rotated from the hot to the cold room and back allowing three days at each temperature on each cycle so that the overall mean temperature was similar for the birds of the changing and the normal regimens during experimental periods. In one experiment three diets with different energy and protein levels were used. The metabolizable energy per kilogram and percent protein were s follows: 3200 and 20 (control diet), 3200 and 17 (low protein diet), and 2800 and 20 (low energy diet). In two other experiments either the control diet or both the control and low energy diets wereused. The response to the different diets was similar at all temperatures so that no diet X temperature interaction occurred. Heat production as a percentage of metabolizable energy intake was least for birds kept at the normal temperature, intermediate for birds subjected to the changing temperatures, and greatest for birds of the cold and hot temperatures. Also, energy gain as percentage of metabolizable energy intake was greatest for birds housed at the normal temperature. The lower energetic efficiency of birds in the changing temperature as compared to birds in the normal temperature suggested an increase in energy cost for maintaining body temperature when chicks were subjected to a widely fluctuating temperature.