Dilated cardiomyopathy (DCM) is a major cause of morbidity and mortality. Occurrence of myocardial fibrosis is an important event in the ventricular remodeling process, which takes place during DCM. Mast cells are well known inflammatory cells implicated in various biological phenomena. The involvement of mast cells in the development of myocardial fibrosis of DCM in rats after autoimmune myocarditis remains unknown. Nine-week-old male Lewis rats were immunized with cardiac myosin and divided into vehicle treated (group V) and disodium cromoglycate (DSCG), a mast cell stabilizer (24 mg/kg i.p.) treated (group DSCG) groups. The animals were sacrificed after 60 d of immunization. The myocardium was excised and preserved for histopathology and protein analysis. Myocardial levels of transforming growth factor (TGF) β1 and collagen-III were quantified. Staining of mast cells was performed by toluidine blue. A significant correlation was obtained between myocardial fibrosis and cardiac mast cell density. DSCG reduced myocardial fibrosis besides preventing infiltration and degranulation of mast cells. Our findings confirm the active participation of mast cells in the progression of myocardial fibrosis in rats with postmyocarditis DCM.