Aldosterone has been implicated as one of the mediators of cardiovascular injury in various diseases. This study examines whether aldosterone antagonism ameliorates or prevents the adverse cardiac effects of hypertension and aging. Male, 22 week old spontaneously hypertensive rats (SHR) were divided into two groups, 15 rats in each. One group received no treatment, the other was given eplerenone (~100 mg/kg/day). At the age of 54 weeks, indices of cardiovascular mass, systemic and regional hemodynamics, including coronary, left ventricular function, and myocardial collagen content were determined in all rats. Hemodynamic studies were done in conscious rats Arterial pressure was lowered only slightly in eplerenone treated rats; and cardiac output and total peripheral resistance did not differ from control rats. Left and right ventricular and aortic mass indices were unaffected by eplerenone; however, concentration of hydroxyproline in the right and left ventricle was decreased significantly (p<0.05) by eplerenone. This was accompanied by an improvement in left ventricular diastolic function and coronary hemodynamics. In conclusion, long-term therapy with the aldosterone antagonist eplerenone ameliorated adverse cardiac effects of both hypertension and aging in SHR. Thus, reduction in myocardial fibrosis paralleled by improvements in left ventricular function and coronary hemodynamics were observed in eplerenone treated SHR.