The fast growth rate and increased body weight of modern broilers makes them susceptible to heart failure and sensitive to heat stress. To find the genetic causes of these problems, gene expression was measured in hearts of two broiler lines that differed in their response to heat stress. Each line was separated into two groups that were kept under heat stress or at ambient temperature. The comparison of gene expression between the two lines and heat treatments suggests that expression of 325 genes were significantly changed in modern broilers under heat stress, while only 3 genes changed their expression in heritage broilers that adapt well to heat stress. Through identification of genes that regulate cardiac response to heat stress in different broiler lines, we can find potential ways to breed heat-tolerant broilers to mitigate both poverty in Africa and the effects of climate change on the global broiler industry