The lining of blood vessels (endothelium) can react to hormones in the blood stream causing the blood vessel muscle to relax (vasodilatation) and allow more blood to flow. The nitric oxide and prostacyclin pathways are well documented in this process. However, evidence points to the existence of a third powerful relaxant called endothelium derived hyperpolarising factor (EDHF) but its identity and mechanism of action have proved elusive. As well as causing blood vessels to relax and more blood to flow, EDHF may be involved in the endothelium signaling, triggering release of a specialised clot dissolving factor called tissue plasminogen activator (t PA). t PA is important to ensure small clots, which are constantly being formed in the circulation, are rapidly dissolved and do not grow large enough to cause heart attacks and strokes.