Abstract Background Interstitial collagen matrix increase and left ventricular (LV) diastolic dysfunction are the most frequent cardiac alterations during aging. Whether changes are part of the aging process or induced by a sedentary life style is not clear. Purpose To evaluate the effects of aerobic exercise on functional capacity, cardiac and myocardial function, and interstitial collagen matrix of aging rats. Methods Male Wistar rats were assigned into three groups: 1) sedentary six-month-old rats (W6, n=24); 2) sedentary 21-month-old rats (W21-Sed, n=25); and 3) exercised 21-month-old rats (W21-Ex, n=20). Physical exercise was by treadmill for 12 weeks. In vivo cardiac evaluation was performed by Doppler echocardiography. Myocardial function was assessed in LV papillary muscle preparation. Total and soluble myocardial collagen concentrations were evaluated by spectrophotometry. Protein expression was analyzed by Western blot. Statistical analysis: ANOVA and Bonferroni or Kruskal-Wallis and Dunn. Results Body weight was greater in W21-Sed and W21-Ex than W6. Before exercise, functional capacity did not differ between aged groups. After training, W21-Ex group ran farther and longer in treadmill than W21-Sed e W6, and W21-Sed presented worse physical capacity than W6. LV diastolic posterior wall, interventricular septum, and relative wall thicknesses were greater in W21-Sed and W21-Ex than W6, and did not differ between aged groups. E wave was lower in both aged groups than W6. E/E' ratio was lower in W21-Ex than W21-Sed. Isolated papillary muscle developed tension was higher in W21-Sed than W6; W21-Ex values did not differ from both W6 and W21-Sed groups. Time to peak tension was higher in W21-Sed and W21-Ex than W6 and did not differ between aged groups. The same results were observed after positive inotropic stimulation. Total collagen concentration was higher in W21-Sed and W21-Ex than W6 and did not differ between aged groups. Soluble collagen did not differ between groups. Type III collagen expression was higher in W21-Sed than W6; the W21-Ex group presented intermediary values that did not differ from W21-Sed or W6 groups. Type I collagen was similar between groups. Conclusion Regular physical aerobic training improves functional capacity and left ventricular diastolic function and attenuates type III collagen myocardial accumulation in aged rats. Acknowledgement/Funding Fapesp, CNPq, Capes, and UNESP