In man and experimental animals, portal hypertension with portal-venous collaterals, is associated with a hyperdynamic circulation, caused by peripheral vasodilatation, mainly in the splanchnic bed. This peripheral vasodilatation is clinically important, since it is thought to be responsible for the pathogenesis of complications of portal hypertension such as ascites, the hepatorenal syndrome and portal hypertensive gastropathy and colopathy. Many cirrhotic patients may not die primarily because of their hepatic dysfunction, but rather because of the consequences of the circulatory abnormalities which are secondary to the liver disease. Circulating hormonal vasodilators from intestinal origin such as glucagon, insufficiently cleared by the liver, are only partly responsible for these changes. Recent experimental data point to a role for an increased production of the locally acting potent vasodilator nitric oxide in the vascular wall, in the pathogenesis of the hyperdynamic circulation. Furthermore, nitric oxide seems to play an important role in the development of portal-venous collaterals. Modulation of the nitric oxide production might offer therapeutic options for the treatment of portal hypertension and its complications.