Mesenchymal stem cells (MSCs) have been shown to attenuate myocardial fibrosis after myocardial infarction by secreting various bioactive molecules that positively affect the failing heart. We hypothesized that serum factors play an active role in the activation of bone marrow MSCs after myocardial infarction and explored the effect of differential exocytosis on cardiac repair after infarct serum preconditioning by examining whether exosomes derived from MSCs have a positive effect on cardiac fibrosis. Bone marrow MSCs were pretreated by collecting rat myocardial infarction serum, followed by the collection of myocardial infarction serum exosomes (MIS-EXO). In vivo, intramyocardial injection of exosomes was performed 30 min after permanent ligation of the anterior descending branches of Sprague Dawley rats, and echocardiography was performed at different time intervals to evaluate cardiac function. Hearts were sampled 4 weeks later, and the degree of myocardial fibrosis and inflammatory response were evaluated using hematoxylin and eosin and Masson's trichrome staining. Treatment with common culture-derived exosomes (CON-EXO) improved cardiac function and myocardial fibrosis after myocardial infarction in rats compared with the myocardial infarction group. In vitro, the antifibrotic effects of different exosomes on tumor growth factor-β-induced fibroblast fibrosis model were assessed by protein blotting, qPCR, and immunofluorescence. Compared with CON-EXO, MIS-EXO exerted superior therapeutic effects in terms of anti-inflammation, improvement of left ventricular function, and reduction of fibrosis. Infarct serum pretreatment with bone marrow mesenchymal stem cell-derived exosomes enhances the anti-cardiac fibrosis effect in rats after myocardial infarction.© 2025. The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.