Action of glucocorticoids on vascular permeability is well established. However, little is known about the action of mineralocorticoids on structure and function of blood vessels.Endothelial cells are targets for both glucocorticoids and mineralocorticoids in the human organism. We combined permeability assays and atomic force microscopy to investigate functional and structural changes in response to both types of steroids. Experiments were performed in primary cultures of human umbilical vein endothelial cells. Aldosterone and dexamethasone were applied for 3 days in culture followed by measurements of transendothelial ion and macromolecule permeability, apical cell surface and cell stiffness. Cell stiffness was measured using the AFM scanning tip as a mechanical nanosensor. We found that aldosterone increased both apical cell surface and apical cell stiffness significantly while transendothelial permeability remained unaffected. In contrast, dexamethasone decreased ion and macromolecule permeability significantly while apical cell surface and cell stiffness did not change. Specific receptor antagonists for dexamethasone (RU486) and aldosterone (spironolactone) prevented the observed responses. We conclude: Glucocorticoids strengthen cell-to-cell contacts ('peripheral action') while mineralocorticoids enlarge and stiffen cells ('central action'). This could explain dexamethasone mediated retention of fluid in the vascular system and endothelial dysfunction in states of hyperaldosteronism.