Hypotension is one of the major complications of hemodialysis contributing to the patient morbidity and mortality and requires aggressive resuscitative measures and sometimes premature termination of hemodialysis. Nitric oxide (NO) is a powerful vasoactive agent that contributes to the regulation of blood pressure. Excessive production of NO by vascular smooth muscle cells, by platelets or by vascular endothelial cells as a result of heparin anticoagulant has been suggested in the pathogenesis of cardiovascular instability in dialysis patients.NO has been found to promote formation of cGMP which causes vasodilatation and inhibition of the production of endothelin-1. So NO production stimulated by heparin may play a role in vasodilatation and thereby lead to hypotensive episodes during hemodialysis.