Cardiac decorin and Type I collagen expression and collagen fibrosis were examined in a chicken line exhibiting the low score normal (LSN) genetic muscle weakness and compared to normal cardiac muscle at 14 and 20 d of embryonic development and at 1 and 6 wk posthatch. These extracellular matrix proteins were selected due to their role in regulating tissue elasticity, which is critical for normal cardiac function. Beginning at 1 wk posthatch, relative levels of decorin were higher in the LSN cardiac muscle than in the control, indicating a time-dependent increase in decorin expression. Type I collagen levels appeared to be unaffected by the LSN condition. Morphologically, myocardial and coronary artery fibrosis did not differ between the control and LSN at all stages examined. These results are suggestive of extracellular matrix remodeling in LSN cardiac muscle that is mediated through a decorin-dependent pathway.