Endothelial cells synthesize and store von Willebrand factor. We have studied the storage and secretion of von Willebrand factor in cultured human umbilical vein endothelial cells. In particular, we were interested in the nature of the storage compartment and the effects of perturbation on the storage and secretion processes. The storage compartment for von Willebrand factor was isolated from homogenates of endothelial cells. By an immunostaining technique the isolated vesicles stained for von Willebrand factor. The staining pattern was similar to that of Weibel-Palade bodies in intact endothelial cells. We concluded that the storage compartment containing von Willebrand factor is identical to the Weibel-Palade body. The von Willebrand factor of the isolated storage vesicles is predominantly constructed of polypeptide chains with a Mr of 220 kD. On the other hand, von Willebrand factor continuously secreted by endothelial cells is constructed of both a 220 kD and a larger precursor (apparent Mr of 275 kD) subunit. The storage vesicles contain von Willebrand factor that supports ristocetin-induced platelet aggregation. Thus, endothelial cells store fully processed, biologically active von Willeband factor within Weibel-Palade bodies. Short-term ( < 1 h) treatment of endothelial cells with the perturbing phorbol ester 4β-phorbol-12-myristate-13-acetate (PMA) results in release of cellular stored von Willebrand factor. 24–48 h after exposure to PMA the endothelial cell distribution of von Willeband factor is changed distinctly. While the contents of the von Willebrand factor storage sites in the cells are gradually restored within 48 h, enhanced amounts of von WiUebrand factor are secreted into the medium. The number as well as the size of von Willebrand factor storage granules in the endothelial cells increase after exposure to phorbol ester, as determined by immunofiuorescence microscopy. Phorbol ester treated cells release stored von Willebrand factor 48 h after they have been stimulated. PMA decreases the von Willebrand factor contents of the extracellular matrix; the deposition of von Willebrand factor in the subendothelium is blocked by PMA, whereas the degradation of matrix von Willebrand factor is not affected. Thus, perturbation of endothelial cells changes the cellular distribution of von Willebrand fact