OBJECTIVE: To study the effect of ulinastatin (UTI) on myocardial ischemia-reperfusion injury (MIRI) through the extracellular signal-regulated kinase (ERK) signaling pathway. MATERIALS AND METHODS: A total of 24 Sprague-Dawley rats were randomly divided into sham group (n=8), I/R group (n=8), and UTI group (n=8), and the rat model of MIRI was established. The changes in the content of serum biochemical indexes, including superoxide dismutase (SOD) and malondialdehyde (MDA). were detected using the kits, and the changes in the expressions of serum inflammatory factors. including interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-alpha), were detected using the quantitative Reverse Transcription-Polymerase Chain Reaction (qRT-PCR) and enzyme-linked immunosorbent assay (ELISA) kits. Moreover, the ERK phosphorylation level in myocardial tissues was detected using the immunofluorescence method, and the ERK phosphorylation level and cleaved caspase-3 expression were detected via qRT-PCR and Western blotting. RESULTS: Compared with those in sham group, the serum SOD content significantly declined. while the MDA content was significantly increased in I/R group, and they were significantly improved in UTI group (p<0.01). The results of detection using qRT-PCR and ELISA kits revealed that the inflammatory factors (IL-6 and TNF-alpha) in UTI group were significantly improved (p<0.01). The immunofluorescence results showed that the ERK phosphorylation level in myocardial tissues was significantly increased in UTI group. The results of qRT-PCR and Western blotting manifested that both ERK phosphorylation level and cleaved caspase-3 expression were significantly improved in UTI group (p<0.01). CONCLUSIONS: UTI can play a protective role in MIRI through up-regulating the ERK signaling pathway.