Aging is associated with progressive structural disorganization of muscular and cardiac fibers, decreasing functional capacity, and increased rates of disease and death. Aging is also characterized by disturbances in protein synthesis with impaired cellular organelle functions, particularly in the mitochondria. The availability of amino acids is a key factor for the overall metabolism of mammals and exogenous supplements of amino acid mixtures (AAm) could be a valid therapeutic strategy to improve quality of life, avoiding malnutrition and muscle wasting in the elderly. We investigated the morphoquantitative effects of long-term AAm supplementation on the mitochondria and sarcomeres (by electron microscope) and on collagen matrix deposition (by histologic techniques) in both skeletal and cardiac muscles of young and aged mice. Our data showed that old animals have fewer mitochondria and massive fibrosis in both muscles. Long-term AAm supplementation increased the number and volume of mitochondria and sarcomeres and decreased fibrosis in both skeletal muscle and hearts in old rats. These findings indicate that AAm restored muscular morphologic parameters and probably improved the mechanical performance of these organs.