This study was carried out at the Poultry Research Farm, Animal production Department, Faculty of Agriculture, Cairo University. The study aimed to investigate the effects of early heat stress on physiological and productive performance of broiler chickens. A total of 235 days old commercial Arbor-Acres broiler chicks were divided into 3 groups. Birds of 1^° and 2' groups were exposed early to heat stress (EHE) where; 1^° group: 71, 3 days old chicks were exposed to heat stress at (42-43°C) for 4 h. 2'^° group: 96, 5 days old chicks were exposed to heat stress at (42-43°C) for 4 h. r^° group: 68 chicks were kept at normal brooding temperature (control). At 8 weeks of age, 30 birds from each of the three mentioned groups (90 chicks) were exposed to heat stress (heat challenged) at 42-43°C for 3 h. Phygologkal responses (rectal temperature, respiration rate, protein and tat) were studied. Performance of Nicks (body weight, growth rate, mortality rate) was recorded and chemical carcass contents were estimated (protein, fat, water and ash contents). The lowest and highest values of rectal temperature and respiration rate, respectively, were recorded in EHE at 3 days of age. Before heat challenge at 8 weeks of age, birds acciimatlzed at 3 days had the highest total plasma protein, albumin and globulin concentrations. Heat acclimatization at 3 days resulted in significant decrease in broilers sodium while plasma potassium significantly increased in 3 or 5 days acolirnatization group at 8 weeks of age. Birds of the control group had significantly higher plasma total lipids. Heat acclimatization at 3 days Caused a significant Increase in body weight at all ages (from 1 to 8 weeks of age). The average daily gain between 1-8 weeks of age showed that birds of EHE at 3 days had significantly the highest average gain. Growth rate from 1-8 weeks of age was significantly higher in 3 days of EHE group. Heat acclimatization at 3 days of age followed by heat stress at 8 weeks of age resulted in reduced water, fat and ash content of carcass meat, while, protein was Increased. EHE at 3 and 5 days of age decreased the mortality rate during the experimental period and during heat challenge at 8 weeks of age.