BACKGROUND The endothelium plays a pivotal role in the control of vascular tone and is responsible for the local release of profibrinolytic factors. Nitric oxide (NO), the original endothelium-derived relaxing factor, and prostacyclin (PGI2) have now been well characterised. The elucidation of their roles in vascular physiology and pathophysiology has been fundamental to recent advances in the treatment and prevention of many cardiovascular diseases. Whilst these factors are of major importance, evidence points to the existence of a third powerful vasodilator called endothelium-derived hyperpolarising factor (EDHF).