This study explored the regulatory mechanism of microRNA-133 (miR-133) secreted from bone marrow mesenchymal stem cell-derived exosome (BMSC-Exo) on myocardial fibrosis and epithelial–mesenchymal transition (EMT) in viral myocarditis (VMC) rats through regulating mastermind-like 1 (MAML1). Here, MAML1 is linked to viral myocarditis.