Refractory hypotension is the main cause of death of patients with septic shock. It has been shown that an excessive release of NO is responsible for the sepsis-induced hypotension and vascular hyporeactivity. Nitric oxide is produced under normal conditions by a constitutive enzyme present, among other cell types, in the endothelial cell, and is necessary for maintenance of normal organ perfusion. Under inflammatory or septic conditions, a new enzyme is expressed in phagocytic cells and vascular smooth muscle cells, giving rise to an uncontrolled NO production that is associated with cytotoxic effects and vasodilatation. Randomized clinical trials have shown that the administration of inhibitors of NO synthesis to patients with septic shock is associated with a greater incidence of shock resolution, without significant adverse effects. The recent discovery of the different biological functions of NO, both under normal and inflammatory conditions, has allowed the development of new concepts about the pathophysiology of septic shock, and has provided the bases to design novel therapeutic strategies for the treatment of septic shock, based on the inhibition of NO synthesis.