Endothelin-1 (ET-1) is a powerful vasoconstricting peptide produced predominantly by vascular endothelial cells, that exerts its effect through the interaction with specific receptors, ETA and ETB, on the underlying smooth muscle cells. Previous studies in normal subjects have demonstrated an increase in forearm blood flow after ET-1 antagonism, suggesting a physiologic role of ET-1 in the regulation of basal vascular tone. However, whether ET-1-mediated tone is increased in hypertensive patients is unknown.