Diminazene aceturate (DIZE) has been reported to enhance the catalytic efficiency of ACE-2 and presumably increases angiotensin 1–7 generation, interfering with cardiac remodeling after myocardial infarction (MI). Our aim was to investigate the chronic effects of DIZE on cardiac dysfunction post-MI. Male Wistar rats underwent myocardial infarction (MI) or SHAM surgery (SO) and were divided into groups treated with DIZE 15 mg/kg/day, s.c. or vehicle (Control). After 4 weeks, the hemodynamic variables were recorded by cardiac catheterism. Hearts were then arrested to obtain the left ventricular (LV) pressure-volume curves in situ. Cardiomyocyte hypertrophy and collagen content were determined by histology. DIZE prevented LV end-diastolic pressure increases in MI rats (MI: 26 ± 3.3 vs. MI-DIZE: 15 ± 1.6 mmHg, P < 0.001) without a significant effect on LV systolic pressure (LVSP). Moreover, DIZE improved LV contractility (+dP/dt, MI: 3014 ± 161 vs. MI-DIZE: 3884 ± 104 mmHg/s, P < 0.001) and relaxation (-dP/dt, MI: −2333 ± 91 vs. MI-DIZE: −2798 ± 120 mmHg/s, P < 0.05). Right ventricular SP was increased in the MI compared to that in the SO group (40 ± 0.6 vs. 30 ± 1.2 mmHg; P < 0.01), and DIZE partially prevented this augmentation. LV stiffness was reduced in MI-DIZE compared with that in MI (0.64 ± 0.01 vs. 0.78 ± 0.02 mmHg/mL; P < 0.01). DIZE treatment reduced the interstitial collagen content by 18% in the surviving LV myocardium. Cardiomyocyte hypertrophy remained unaffected by DIZE treatment. Our findings show that chronic DIZE treatment post-MI attenuates the morphofunctional changes induced by MI in rats. The effects on LV -dP/dt, chamber stiffness and collagen content suggest this drug can be used as a therapeutic agent to reduce interstitial fibrosis and diastolic dysfunction after MI.