Objective:To investigate the effect of inhibitory activity of high mobility group protein B1 (HMGB1), signal transduction and activator of transcription 3 (STAT3) on myocardial ischemia-reperfusion injury in rats.Methods:In vivo and in vitro models of MIRI were established. SD rats were randomly divided into a sham group, a model group, a glycyrrhizic acid group, and a NSC74859 group, with 6 rats in each group. Rats in the sham group were not ligation, and rats in the sham group and model group were not given medication. The rats in the glycyrrhizic acid group and the NSC74859 group were injected with HMGB1 antagonist glycyrrhizic acid or STAT3 inhibitor NSC74859 5 mg/kg in the tail vein at 12 h 30 min before ischemia/reperfusion and 30 min after ischemia, respectively. Left ventricular shortening fraction (FS) and left ventricular ejection fraction (EF) were evaluated by echocardiography, and apoptosis of cardiomyocytes was evaluated by hematoxylin-eosin (HE) and TUNEL staining. The expression levels of HMGB1, STAT3, and phosphorylated STAT3 (p-STAT3) were detected by real-time fluorescence quantitative PCR and Western Blot. The viability of H9C2 cells was determined by the MTS assay, intracellular ATP content was determined, and the mitochondrial membrane potential of H9C2 cells was measured by flow cytometry to evaluate the survival of cardiomyocytes. The action mode of HMGB1/STAT3 was studied by the immunoprecipitation method. The expression and migration of HMGB1/STAT3 in the nucleus and cytoplasm were detected by immunostaining. Results:After inhibiting the expression of HMGB1 or STAT3, EF and FS were increased, and immune infiltration and apoptosis of cardiomyocytes were decreased. Inhibition of HMGB1 expression could decrease the expression of STAT3, but inhibition of STAT3 expression didn’t affect the expression of HMGB1. Hypoxia could lead to increased expression of HMGB1 and p-STAT3, and decreased expression of STAT3. After 8 hours of hypoxia, the expression level of STAT3 suddenly increased. After reoxygenation, the expression of HMGB1 and STAT3 decreased, and the expression of p-STAT3 increased, but p-STAT3 (Ser 727) didn’t participate in this process. After ischemia-reperfusion injury, HMGB1 and STAT3 binded firmly in cardiomyocytes, but inhibition of STAT3 or HMGB1 weakened this binding. Inhibition of HMGB1 or STAT3 expression could reduce myocardial ischemia-reperfusion injury. The expression of HMGB1 in reoxygenated cardiomyocytes increased after hypoxia, and HMGB1 migrated from the nucleus to the cytoplasm.Conclusions:Inhibiting the activity of the HMGB1/STAT3 axis effectively reduces MIRI in rats.