Objective To study the effect of hyperthermia-induced MMP-9 on expression of claudin-1 in in vitro BBB model.Methods BMEC and astrocytes were cultured.An in vitro BBB model was established with its permeability assayed by LY test.The BBB model was randomly divided into 37℃ group,39℃ group,37℃+MMP-9 group and 39℃+MMP-9 group.The permeability of BBB model was recorded at 30,60 and 90 min respectively.Expression of claudin-1 in BMEC was detected by Western blot with immunocytochemical staining.Results The permeability of BBB model was higher in 39℃ group than in 37℃ group (P＜0.05) and was significantly higher in 39℃ +MMP-9 group than in 37℃ group (P＜0.05).The number of claudin-1 positive BMEC was smaller in 39℃ +MMP-9 group than in 37℃ group at the same time points (P＜0.05) and in 39℃ +MMP-9 group than in 39℃ group at 90 min (75.0±12.8/HP vs 97.0±6.1/HP,P＜0.05).The expression level of claudin-1 was lower in 39℃ group than in 37℃ group (P＜0.05) and was significantly higher in 39℃ +MMP-9 group than in 37℃ group (P＜0.05),which further increased with the increasing time.Conclusion Hyperthermia can downregulate the expression of claudin-1 in in vitro BBB model and upregulate the permeability of in vitro BBB model.Adding MMP-9 can further aggrevate the damage of in vitro BBB model,indicating that hyperthermia can aggrevate the damage of in vitro BBB model through MMP-9.