1. This study investigated effects of conditioning at 5 d of age and food restriction during a hot period on responses to prolonged heat stress of two fast (G1 and G2) and one slower-growing (G3) broiler stocks. 2. Chicks from each stock were divided into 3 groups: control, conditioned (chicks exposed to 36°C for 24 h at 5 d of age) and food restricted (during the heat stress food was withdrawn 2 h before the hot period and chicks were fed between 17.00 and 08.00 h). Prolonged heat stress was induced daily from 28 to 49 d by heating until the ambient temperature reached 32° to 35°C between 10.00 and 17.00 h. 3. Body weights, mortality and rectal temperatures were lower and food conversion higher for stock G3 than G1 or G2. 4. Management procedures used in this study improved food conversion and lowered mortality without affecting body weight. Rectal temperatures of the treated groups were similar to those of the controls.