Background/Aims: Diarrhea-associated hemolytic uremic syndrome is associated with the presence of Shiga toxin (Stx1, Stx2 and several variants) in the circulation. The aim of this study is to examine the possible triggering effect of Stx1 on the exocytosis of Weibel-Palade bodies (WPbs). Methods: Cultured human umbilical venous endothelial cells (HUVECs) and glomerular microvascular endothelial cells (GMVECs) were stimulated by thrombin and Stx1 in both static and flowing conditions. The amount of secreted von Willebrand factor (VWF) in the supernatant as well as the remaining intracellular fraction was determined. Results: In HUVECs and in 2 out of 4 GMVECs, the stimulation of Stx1 in flow at 1 dyne/cm<sup>2</sup> resulted in a decrease of intracellular VWF. This is contrary to the results of Stx1 applied in static conditions. At a higher flow rate of 5 dyne/cm<sup>2</sup>, no effect in GMVECs was observed. Conclusion: Stx1 can contribute, via an effect on WPbs, to the exocytosis of WPbs in flow conditions in HUVECs and probably in GMVECs. This results in the release of VWF, suggesting an initiating role of the coagulation system in the pathogenesis.