Climate change will negatively impact the ability to satisfy global nutritional needs. The objective of this study was to characterize the effect of genetics (breed), embryonic thermal conditioning and post-hatch heat stress on physiological response. We tested a unique inbred Fayoumi line indigenous to Egypt, an outbred broiler breeder male line, and an F18 broiler-Fayoumi advanced intercross line (AIL). About 1000 chicks were hatched from eggs incubated at either normal (37 C) temperatures throughout, or elevated temperatures (39 C) for part of the incubation period. From d 22 to d 28 after hatch, chicks were either heat stressed (35 C for 7 hr/d) or maintained at constant 22 C (“control”). Body temperature and blood parameters were measured before thermal stress, and after 4 hr and 6 d of thermal stress. Both egg treatment and chick treatment significantly affected bodyweight at d7, 14, 21 and 28, and the change in body temperature from pre-heat stress levels; however, there were no interactions between the egg and chick treatments, suggesting that the mechanisms of response to thermal stress in embryonic and post-hatch periods are relatively independent. Egg treatment did not affect blood parameters, but heat-stressed chicks had significantly lower blood hemoglobin, hematocrit, carbon dioxide partial pressure, and higher pH and saturated oxygen, than control chicks. There were several interactions of genetic line with heat treatment effects. Data and tissue samples from this study form the foundation for a major investigation on the effects of genetics and thermal conditioning on heat resistance in chickens.