Background The release of Weibel-Palade Bodies (WPB) is a form of endothelial cell activation But the signal transduction pathway leading to WPB release is not yet defined We hypothesized that small G-protein rac1 and reactive oxygen species (ROS) mediate the ligand induced release of Weibel-Palade Bodies Methods We tested this hypothesis by using wild-type and mutant adenoviral rac1 expression vectors, and by manipulating the production and destruction of superoxide and hydrogen peroxide in human aortic endothelial cells (HAEC) KH*2/5DResults Thrombin (1 0 Unit, 30 min) induced the increase of WPB release by 3 7-fold in HAEC, and that H 2O 2 (0 1 mmol/L, 30 min) induced by 4 5-fold These results correlated with thrombin-stimulated activation of rac-GTP binding activity by 3 5-fold, and increase of ROS production by 3 4-fold The dominant negative adenoviral rac-N17 gene transfer dramatically inhibited the release of WPB by 64 2% (control) and 77 3% (thrombin-stimulation), and decreased ROS production by 65 5% (control) and 83 6% (thrombin-stimulation) compared with non-infected cells, respectively Anti-oxidants, catalase and N-acetyl-cysteine significantly decreased the release of WPB by 34% and 79% in control cells, and further decreased by 63 6% and 46 7% in rac-N17 transferred cells compared with non-infected cells We also confirmed that rac1 was located upstream of ROS in the WPB release pathway KH*2/5DConclusions Small G-protein rac1 medicates ligand-induced release of Weibel-Palade Bodies in human aortic endothelial cells, and the signal pathway of WPB release is a rac1-dependent ROS regulating mechanism Background The release of Weibel-Palade Bodies (WPB) is a form of endothelial cell activation But the signal transduction pathway leading to WPB release is not yet defined we hypothesized that small G-protein rac1 and reactive oxygen species (ROS) mediate the ligand induced release of Weibel-Palade Bodies Methods We tested this hypothesis by using wild-type and mutant adenoviral rac1 expression vectors, and by manipulating the production and destruction of superoxide and hydrogen peroxide in human aortic endothelial cells (HAEC) KH * 2/5 Results of Thrombin ( 1 0 Unit, 30 min) induced the increase of WPB release by 3 7-fold in HAEC, and that H 2O 2 (0 1 mmol / L, 30 min) induced by 4 5-fold These results correlated with thrombin- stimulated activation of rac-GTP binding activity by 3 5-fold, and increase of ROS production by 3 4-fold The dominant negative adenoviral rac-N17 gene transfer mutant inhibited the release of WPB by 64 2% (control) and 77 3% (thrombi n-stimulation), and decreased ROS production by 65 5% (control) and 83 6% (thrombin-stimulation) compared with non-infected cells, respectively anti- oxidants, catalase and N- acetyl- cysteine ​​significantly decreased the release of WPB by 34% and 79% in control cells, and further decreased by 63 6% and 46 7% in rac-N17 transferred cells compared with non-infected cells We also confirmed that rac1 was located upstream of ROS in the WPB release pathway KH * 2 / 5DConclusions Small G-protein rac1 medicates ligand-induced release of Weibel-Palade Bodies in human aortic endothelial cells, and the signal pathway of WPB release is a rac1-dependent ROS regulating mechanism