An experiment was designed to evaluate the respective effects of sex and body condition (Heavy, Light) and their interaction with acclimatizing. A total of 64 broilers were used, with 32 males and 32 females including 16 heavy (average 2096g at day 36) and 16 light (average 1632g at day 36) animals within each sex. From Day 36 to Day 40, half of the animals were raised in hot environment (TA = 31.5°C during day) and half in temperate environment (TA=25.1°C during day). In day 42, there was a heat challenge during which all the animals were all submitted to high ambient temperature (TA=35.0°C). Parameters recorded included body temperature (BT, °C) and hyperventilation rate (HV, Insp/min). During heat challenge, the effect of the previous exposure to climate was significant, with a lower BT (43.14°C vs 43.74°C, p<0.01) and NH (140.5insp/min vs 150.5insp/min, p<0.05) of animals from hot environment. The effect of body condition was not significant whereas sex was significant (BT: 43.73°C vs 43.19°C, p<0.05 and NH: 132.5insp/min vs 149.5 insp/min, p<0.05 for males and females respectively). During heat stress, 18 animals died (14 non acclimatized and 4 acclimatized). The analysis of the relationship of death status with other parameters showed that effect of acclimatizing and sex on mortality were significant (p<0.05) whereas body condition was not significant.