Nitric oxide is involved in the regulation of the circulation in physiologic and pathophysiological conditions. Evidence indicates that alterations in endothelial production of nitric oxide may be involved in the pathogenesis of central hypertension, pulmonary hypertension, renal disease and coronary vasopastic disorders. In addition to being involved in regulation of the circulation in physiologic and pathophysiologic conditions, the inducible form of the enzyme may play a role in the refractory hypertension.