Nitric oxide (NO) is now considered as the endogenous nitrovasodilator which is mainly derived from vascular endothelial cells in physiological conditions. Biosynthesis of NO is controlled by a family of enzymes, the NO synthases (NOS), that can be divided into two major subgroups, namely the constitutive and the inducible NOS. The constitutive NOS is the principal isoform found in endothelial cells. Endothelial dysfunction, as seen in chronic hypoxic lung diseases, impairs endogenous production of NO, thereby causing and/or aggravating pulmonary hypertension. A logical means to reduce pulmonary hypertension would consist in supplying the patients with exogenous NO. Given by inhalation, NO is a selective pulmonary vasodilator, as it rapidly combines with haemoglobin, which inactives NO, and therefore prevents the occurrence of systemic hypotension. Increased production of NO resulting from induction of the inductible isoform of NOS also occurs in inflammatory disorders of the lung and the airways. Henceforth, it is possible to detect, and quantify, increased endogenous pulmonary synthesis of NO by measuring it in the exhaled air. Measuring exhaled NO could, therefore, represent a safe means to detect bronchial inflammation in patients. However, the importance of endogenous NO in the modulation of bronchial tone remains to be established. Current investigations include studies looking at regulatory mechanisms of cellular expression of various NOS isoforms on the one hand and, on the other hand, clinical evaluation of short- and long-term inhalation of NO in patients with primary and secondary pulmonary hypertension.