Summary. Studies on the effect of DDAVP both in vitro and in vivo are reported. In order to define the extent of the DDAVP induced rise of circulating endothelial cell proteins in normal individuals and the endothelial cell defect in von Willebrand's disease (vWd) we have measured the effect of intravenous DDAVP on a range of possible endothelial cell markers in normal subjects and in patients with mild haemophilia and vWd. In a series of double blind cross over studies on normal volunteers we have tested the effect of naloxone, DDAVP or saline on circulating levels of factor VIII related activites (VIIIR) and plasminogen activator (PA). The results confirmed the effect of DDAVP on circulating levels of VIIIR and PA but showed that it did not induce release of these activities from cultured endothelial cells in vitro nor did it influence circulating levels of other endothelial cell markers including fibronectin, antithrombin III and platelet factor 4. Infusion of nalaxone did not significantly after circulating levels of VIIIR or PA nor the response of these to DDAVP suggesting that normally these activities are not subjected to a vasopressin drive.