Abstract: 【Abstract】 Objective To determine the plasma level of mitochondrial DNA (mtDNA) and the expression of Toll like receptor-9/mitogen-activated protein kinase (TLR-9/MAPK) in the brain tissues of rats with subarachnoid hemorrhage (SAH). Methods Sprague-Dawley rats were used to establish SAH models with a modified method of endovascular perforation. Twenty four hours after the establishment of the models, the plasma level of mtDNA in the rats was detected by real-time PCR; the expressions of TLR-9 and P38 MAPK in the brain tissues were detected by reverse transcription PCR (RT-PCR) and immunohistochemical staining. Differences between the SAH (n =20) and sham-surgical (n =20) groups were compared. Results Rats in the SAH group had higher levels of plasma mtNDA and expressions of TLR-9 and P38 MAPK in brain tissues than those in the sham-surgical group (P <0.05). Conclusion Plasma free mtDNA and expressions of TLR-9/MAPK increase simultaneously in rats with SAH. mtDNA may induce system and local inflammation reactions in brain.