Angiotensin converting enzyme2 (ACE-2) is a newly described enzyme, with antagonistic effects to those of the classical ACE (ACE-1). Both angiotensin II (AngII) and aldosterone play an important role in the pathophysiology of congestive heart failure (CHF), and in the adverse cardiac remodeling during its development. In this study, we examined the effects of experimental CHF induced by an aortocaval fistula (ACF), and of its treatment with AngII and aldosterone inhibitors, on the relative levels of ACE-1 and ACE-2. We also compared the effects of spironolactone, an aldosterone antagonist, and eprosartan, an AngII receptor antagonist, on heart hypertrophy and fibrosis in rats with ACF. Spironolactone (15mg/Kg/d ip, via minipump) or eprosartan (5mg/Kg/d ip via minipump) were administered into rats with ACF for 14 and 28 days. Specific antibodies were used to determine the protein levels of myocardial ACE-1 and ACE-2. ACF increased the cardiac levels of ACE-1 and decreased those of ACE-2. Heart-to-body weight ratio (HW/BW) significantly increased from 0.30±0.004% in sham controls to 0.50±0.018% and 0.57±0.006% (p<0.05) in rats with ACF, 2 and 4 weeks following surgery, respectively, in association with increased plasma levels of aldosterone. The area occupied by collagen increased from 2.32±0.27% to 6.90±0.64% and 8.03±0.93% (p<0.05) 2 and 4 weeks following ACF, respectively. Both spironolactone and eprosartan decreased cardiac mass and collagen content, and reversed the shift in ACE isoforms. ACF alters the ratio between ACE isoforms, in a manner that increases local AngII and aldosterone levels. Early treatment with both AngII and aldosterone antagonists is effective in reducing this effect. Thus, ACE isoform shift may represent an important component of the development of cardiac remodeling in response to hemodynamic overload, and its correction may contribute to the beneficial therapeutic effects of RAAS inhibitors.