Regardless of the origin, injury to the heart can result in cardiomyocyte hypertrophy, fibrosis, and cell death. Myocarditis often progresses to dilated cardiomyopathy (DCM), a major cause of heart failure. In our study, we used a rat model of myosin-induced experimental autoimmune myocarditis (EAM), in which the heart transits from an acute phase (inflammatory myocarditis) to a chronic phase (remodeling and DCM). Our objective was to investigate whether T-3999, a novel phenylpyridazinone, can reduce this progression. Four weeks after myosin injection, T-3999 was administered daily to male Lewis rats in two doses (3 and 10 mg/kg, orally). Four weeks later, treatment was terminated; hemodynamic and echocardiographic measurements were performed; hearts were excised for histopathology and estimation of histamine, mRNA, and protein levels. Mortality rate was reduced by drug treatment. T-3999 reduced % fibrosis and tissue collagen III. Profibrotic markers—transforming growth factor-β₁, tumor necrosis factor-α, and galectin-3—were attenuated by treatment. Mast cell density and degranulation, and tissue histamine concentration were also reduced. This indicates an anti-inflammatory effect of the drug in reducing fibrosis. Hypertrophy was reduced as reflected by reduced myocyte diameter and natriuretic peptide expression. T-3999 treatment increased the sarcoendoplasmic reticulum Ca²⁺ ATPase 2 protein level and improved several cardiac function parameters. The reduction of the remodeling process and improvement in myocardial function suggest an effect of T-3999 in attenuating ventricular remodeling in post-myocarditis DCM.