This experiment was conducted at Kharabo farm, Faculty of Agriculture, University of Damascus. A total of 252 chicks of Japanese quail breeder females were distributed on the first day of life into two groups, Gicks in G1 were reared from 1-28 days of age under ideal required temperature according to their age, while chicks in G2, were subjected to heat stress from 8 am to 5 pm at 42 °C from the first day of hatching until 28 day of age and to the ideal required temperature from 5 pm until 8 am. Chicks of each group were distributed into three subgroups, with three replicates per subgroup.