The endothelium modulates vascular tone by the release of constricting and relaxing substances that act on the underlying smooth muscle. This regulatory activity of the endothelium is dysfunctional in a number of cardiovascular conditions, including essential hypertension and hypercholesterolemia. Previous studies from our group have implicated a decreased action of endothelium-derived nitric oxide (NO) as the mechanism responsible for this abnormality. Whether this reduced bioactivity of NO is related to vasoactive prostanoids remains uncertain.