Maintaining the proportions of the myocytes, interstitium and collagen, is fundamental for myocardial function,  This study examined the changes produced by aging in the densities of myocites, interstitium and collagen of the left ventricular myocardium male Wistar rats and to see whether resistance exercise may influence these changes. Three groups of animals were used:  Control Group, sacrificed at 13 months of age, Sedentary Group, sacrificed at 16 months of age, and Trained Group, sacrificed at 16 months of age. The volume densities (Vv) of cardiac myocytes, interstitium, and collagen were determined in histological sections of the left ventricular wall. It was concluded that aging decreased the area occupied by myocytes with an increase in the area occupied by the interstitium and collagen. Resistance exercise promoted the reversal of collagen accumulation, not influencing the effects of aging in the space occupied by myocytes and interstitium in the myocardium.