Mast cell chymase (CHY), a multifunctional protease with Ang II-forming activity, garnered interest due to the success of angiotensin converting enzyme inhibitors (ACEi) in the treatment of heart failure. The VALIANT trial evaluated the contribution of this alternate Ang II pathway by adding an AT1 Ang II receptor blocker (ARB) to an ACEi post-myocardial infarction (MI), but the combination showed no additional benefit, suggesting CHY was unimportant post-MI. However, the role of CHY, independent of its Ang II-forming activity, remains unaddressed. To test this, we first re-evaluated the role of CHY-generated Ang II by blocking the effects of ACE+CHY-generated Ang II using an ACEi+ARB+AT2 receptor blocker (PD122139) combination (AAA), and compared this to ACEi monotherapy in wild-type (WT) mice. We then addressed the Ang II-independent role of CHY by comparing these therapies in chymase (MMCP-4)-deficient (KO) vs WT mice. WT and KO mice (N=10-20) underwent sham or MI (90-min ischemia-reperfusion) surgery, and were treated daily with vehicle (Veh), ACEi, or AAA 24h post reperfusion. After 14 days, echocardiographic and hemodynamic analyses indicated no significant inter-genotype differences in sham animals. In post-MI WT mice, ACEi vs Veh resulted in improved ejection fraction (EF) (34±1.8% vs 28±1.7%; p<0.05) and reduced LV end-diastolic dimension (LVEDD) (4.2+0.09 mm vs 4.7±0.1 mm; p<0.05). However, AAA did not result in further improvements in either parameter as compared to ACEi, suggesting that CHY-generated Ang II is unimportant post-MI, confirming VALIANT. While the initial (24-h post-MI) infarct sizes were similar between genotypes (WT: 63±5% vs KO: 65±2% infarct as percent of area at risk; p=0.6), EF was superior in KO vs WT mice in all treatment groups (36±3% vs 28±2% (Veh); 47±3% vs 34±2% (ACEi); 49±3% vs 36±3% (AAA); p<0.05). Further, LV dilatation was also more pronounced in WT mice (LVEDD: 4.7±0.4 mm, WT-Veh vs 4.3±0.5 mm, KO-Veh; p<0.05). ACEi decreased mean arterial pressure post-MI (p<0.01), but not differentially in WT vs KO (Veh: 70±5 mmHg vs 66±3 mmHg; p=0.47; ACEi: 48±3.7 mmHg vs 38±4.8 mmHg; N=6−10; p=0.1). Thus, we conclude that CHY, independent of Ang II, is a potential therapeutic target for the treatment of the post-MI heart.