Aldosterone is a key regulator of salt and water balance in humans. Besides kidney blood vessels are targets for this hormone. Human endothelium responds to aldosterone in few minutes with epithelial sodium channel-mediated cell swelling. This fast action cannot be explained by genomic activation. To search for putative membrane receptor sites we attached single aldosterone molecules were via 6 nm spacers to the tip of an atomic force microscope. The tip was brought into contact with the apical plasma membrane of living human endothelial cells (EAhy926) maintained in buffer at 37UC. If aldosterone was bound to a surface receptor, lifting the AFM tip off the cell surface should bend the tip cantilever until the bond ruptures. This signal can be quantified. We found that about 1 out of 5 endothelial cells exhibited aldosterone surface binding (rupture force: 150 to 300 pN). Binding probability was about 30 to 50%. When aldosterone (100 nM) was added to the bath solution, binding probability dropped below 10%. When dexamethasone (100 nM) was added to the bath, binding probability remained unaffected. Application of the classical aldosterone receptor antagonist spironolactone to the bath solution (100 nM) had no effect on surface aldosterone binding. We conclude: Human endothelial cells exhibit apical plasma membrane surface receptor sites of high aldosterone affinity. The putative plasma membrane receptors are specific for aldosterone and different to the classical cytosolic receptors.