PHYSIOLOGICAL EFFECTS OF NITRIC OXIDE: The generation of nitric oxide by the vascular endothelium maintains a vasodilator tone that is essential for the regulation of blood flow and pressure. In the brain, nitric oxide acts as a mediator of cell-cell signalling. In the peripheral nervous system nitric oxide is also released from many nerves previously classified as non-adrenergic and non-cholinergic. Thus this simple gaseous molecule performs a wide variety of physiological functions. POTENTIAL FOR THERAPEUTIC MANIPULATIONS: Impaired production of nitric oxide can be countered by the administration of nitric oxide donors (in hypertension, atherosclerosis, gastrointestinal and genitourinary disorders) or by inhalation of nitric oxide gas (in chronic pulmonary hypertension or adult respiratory distress syndrome). The biggest challenge is to develop strategies that target the cytotoxic and damaging actions of nitric oxide without interfering with its essential protective functions.