To define the impact of age on microscopic structural remodeling after myocardial infarction, the physical properties of infarct scar tissue and viable remote zone tissues in young (3 months) and older adult (18 months) Fischer rats were quantified with the use of high-frequency (50 MHz) high-resolution acoustic microscopy 3 months after coronary artery occlusion. We observed that integrated backscatter increased by 100% in the viable zones of old animals after infarction, but remained relatively unaffected in the same regions of younger animals. Mathematical models of myocardial scattering behavior indicated that a 25% increase in stiffness of the extracellular matrix materials in viable zones likely occurred in the older animals. Alterations in gross tissue collagen content were not responsible for this increased stiffness. These observations are compatible with the hypothesis that progressive age-related changes in the quality of the collagen (e.g., excessive age-related crosslinking) rather than its amount per se may have altered the stiffness of the extracellular matrix of remodeled viable tissue in older animals.