Young and senescent rats were exposed to audiogenic stress for 6 h running and the effects of the treatment on the atrial myocardium were evaluated. In both young and senescent animals subcellular alterations of atrial cardiomyocytes were seen, consisting mainly of changes at the mitochondrial level. In particular, sections from young treated rats exhibited swollen mitochondria with lysis of their cristae and dissolution of the matrix. An enlargement was observed of the space between the outer and inner membranes of the mitochondria, which also showed an increase in size. In addition, aged treated specimens revealed decreased density of the sarcoplasm and marked heterogeneity in the myofibril pattern with ruptures and loss of continuity of the myofilaments.