Nitric oxide is a newly discovered gas that mediates communication between adjacent cells and signals inside cells. Defects in the nitric oxide signalling cascade have been found in patients with systemic hypertension, pulmonary hypertension, and coronary artery disease. High concentrations of nitric oxide are cytotoxic. Over-production of nitric oxide contributes to the pathogenesis of septic shock and uremic bleeding. This review focuses on the cardiovascular and platelet actions of nitric oxide, with emphasis on clinical studies of agents that modulate the nitric oxide signalling cascade.