At 5 days of age, one group of male broiler chicks was exposed to thermal conditioning (38 A?C, 42 % humidity for 24 hours) while the other group was not. From 11 to 41 days of age, temperature was diurnally cyclic: 26 A?C (1600 to 0900 h) and 30 A?C (0900 to 1600 h). From day 19, diets were control or dual feeding of a high protein fraction (1600 to 0900 h) and an energy-rich fraction (0900 to 1600 h). Growth and feed efficiency were slightly enhanced by thermal conditioning (+ 4 %) and slightly reduced by dual feeding (- 4 %). During the thermal challenge, mortality was reduced by both factors. Prior to the thermal challenge period, thermal conditioning always reduced body temperature. During the thermal challenge period, body temperature was reduced by thermal conditioning and by dual feeding. Body temperature measurements taken at 32 days of age gave an estimation of the ability of the chickens to cope with heat stress 2 days later. For thermal conditioned chicks fed a control grower diet, pectoral mass was increased. The liver and gizzard were significantly heavier for thermal conditioned and non-conditioned chicks on a dual feeding program. This suggests that thermal conditioning at 5 days of age induces a consistent metabolic change in broiler males. The dual feeding program may have induced transitory effects on heat production during the heat stress.