Objective To investigate the effect of exosomes (Exo) derived from bone marrow mesenchymal stem cells (BMSCs) on rats with renal ischemia/reperfusion (I/R) injury, and to explore the underlying mechanism. Methods Forty male SD rats were randomly divided into four groups：Sham group, I/R group, I/R+Exo group and I/R+Exo antagomir group. The rats in I/R+Exo group were injected with exosomes through the tail vein, and then the renal I/R operation was conducted. The rats in I/R+Exo antagomir group were injected with exosomes transfected with miR-145-5p antagonist through the tail vein, and then the renal I/R operation was conducted. The rats in I/R group were injected with the same amount of phosphate buffer saline (PBS) solution through the tail vein, and then the renal I/R operation was conducted. The rats in Sham group were injected with the same amount of PBS solution through the tail vein, and then the abdominal cavity was opened and closed without vascular clipping. Twenty-four hours after reperfusion, renal tissues and carotid blood were collected, pathological changes of renal tissues were observed, blood urea nitrogen (BUN) and serum creatinine (Scr) were detected, the mRNA expression of miR-145-5p was determined, the apoptosis rate and the cleaved caspase 3 expression were measured, the superoxide dismutase (SOD) activity, malonaldehyde (MDA) content and reactive oxygen species (ROS) generation were ascertained, and the expression levels of inflammatory factors, including inducible nitric oxide synthase (iNOS), interleukin-6 (IL-6), tumor necrosis factor-α (TNF-α) and nuclear factor-κB (NF-κB) were detected. Results Compared with the Sham group, the renal tissues of I/R group and I/R+Exo antagomir group showed glomerular necrosis and hypertrophy, with tubule dilation and casting formation. Compared with the I/R group, the renal tissues of the I/R+Exo group showed significantly relieved glomerular necrosis and hypertrophy, and markedly reduced tubule expansion and casting formation. The renal tissue injury scores of the I/R group and I/R+Exo antagomir group were higher than those of the Sham group and I/R+Exo group (all P<0. 05), and the renal tissue injury score of I/R+ Exo group was higher than that of Sham group (P<0. 05). Serum BUN and Scr levels in the I/R group and I/R+Exo antagomir group were higher than those in the Sham group and I/R+Exo group (all P<0. 05) . The expression of miR-145-5p in the I/R group and I/R+Exo antagomir group was lower than that in the Sham group and I/R+Exo group (P<0. 05), and the expression of miR-145-5p in the Sham group was lower than that in the I/R+Exo group (P<0. 05). The apoptosis rate and cleaved caspase 3 expression in the I/R and I/R+Exo antagomir groups were higher than those in the Sham and I/R+ Exo groups (all P<0. 05), and the apoptosis rate and cleaved caspase 3 expression in the I/R+Exo group were higher than those in the Sham group (both P<0. 05). The SOD levels in the I/R group and I/R+Exo antagomir group were lower than those in the Sham group and I/R+Exo group (all P<0. 05). MDA content and ROS production in the I/R group and I/R+ Exo antagomir group were higher than those in the Sham group and I/R+Exo group (all P<0. 05). The levels of iNOS, IL6, TNF-α and NF-κB in the I/R group and I/R+Exo antagomir group were higher than those of the Sham group and I/R+ Exo group (all P<0. 05). Conclusion BMSCs-derived exosomes can alleviate renal I/R injury in rats, and the mechanism may be related to the inhibition of oxidative stress and inflammation in renal tissues through exosomes' delivery of miR-145-5p. [ABSTRACT FROM AUTHOR]