Tight junction is a crucial structure in the control of paracellular transport across epithelial/endothelial barriers. This study investigated the protective effect of quercetin against hydrogen peroxide (H(2)O(2))-induced tight junction disruption and hyperpermeability in ECV304 monolayers. Nonlethal concentration of H(2)O(2) (100 micromol/L; 4 hours) decreased expression of the tight junction proteins zonular occudens (ZO)-1 and occludin as well as disrupted the junction structure at the cell border. Concurrently, the increased activities of extracellular signal-regulated kinase (ERK)1/2 and p38 mitogen-activated protein kinase (MAPK) were observed. Pretreatment with quercetin (10 micromol/L; 30 minutes) prior to H(2)O(2) prevented the loss of ZO-1 and occludin. In addition, the level of phosphorylated p38 MAPK, but not that of the phosphorylated ERK1/2, decreased in quercetin-pretreated group. These findings suggested that the protective effect of quercetin involved the inhibition of phosphorylated p38 MAP activity. Furthermore, quercetin could also preserve the functional integrity of ECV304 monolayers from H(2)O(2) exposure.