Chickens selected for meat production (broilers) have also acquired deleterious traits, including high susceptibility to heart failure. Our group has been comparing a modern broiler line, Ross708, with a Heritage line that grows with properties similar to broilers from the 1950s. Morphological studies revealed that the normalized heart mass of the Ross708 and Heritage birds are similar until 14 days post-hatch. After day 14, the relative heart mass of the Ross line decreases in comparison with the Heritage line. One interpretation of these data is that cardiac post-hatch development is similar between the two lines until day 14; after that time point selection for rapid skeletal muscle growth may have limited cardiac muscle growth in the Ross 708 line. It is possible that identifying differences in gene expression patterns between the Ross708 and Heritage line may help identify genes that have been affected by selection for modern broilers. We are currently using high throughput sequencing to compare the transcriptome of cardiac samples taken at different time points post-hatch from the Ross708 and Heritage lines. Preliminary analysis indicates that at 42 days post-hatch, ventricle tissue from Ross708 birds are expressing higher levels of genes that may generate fibrotic deposits compared with the Heritage birds. In addition, we are evaluating the impact of heat stress on the post-hatch cardiac gene expression in these two lines.