The aim of this study was to evaluate the effect of insulin on the release of vWf in vivo during an oral glucose tolerance test (OGTT) performed in normal, glucose-intolerant and diabetic subjects and in vitro on human endothelial cells. Twenty-eight subjects exhibiting risk factors for diabetes underwent an OGTT: 11 subjects proved to be normal, 7 were glucose-intolerant and 10 diabetic. In each group, the vWf and PAI-1 plasmatic levels were measured at t = 0, 30 min and 180 min after the beginning of the test. Human endothelial cells from non-diabetic and diabetic subjects were incubated in the presence of human insulin at various concentrations (0.25, 2.5, 25 and 250 mUI/ml). After 1, 4, and 24 hours of incubation, the release of vWf and endothelin 1 was measured in the cell supernatant and the intracellular amount of vWf in the cell lysate. During the OGTT, the vWf levels in plasma were not affected despite a 4.5-, 6-, and 2.5-fold increase in insulin levels in normal, glucose-intolerant and diabetic subjects, respectively. Although raised in all three groups, PAI-1 plasmatic levels remained constant during the test. After 24 hours of exposure to insulin (0.25 mU/ml), the release of vWf by endothelial cells reached 35.94 +/- 23.08 % of the basal value for non-diabetic subjects, and 27.57 +/- 10.05 % for diabetic patients. Similar results were observed in non-stimulated cells. Insulin had no influence on intracellular vWf content, which remained comparable to control values. Regardless of its concentration, insulin failed to stimulate the release of vWf by endothelial cells of non-diabetic and diabetic subjects, while its ability to stimulate the release of endothelin 1 was preserved. In conclusion, hyperinsulinemia had no adverse effect on circulating vWf in normal or diabetic subjects. Neither release nor intracellular vWf content in non-diabetic or diabetic endothelial cells was influenced by insulin in vitro.