Endogenously produced nitric oxide (NO) is detectable in the exhaled air of resting humans, and the amount of exhaled NO increases during exercise. It is believed that NO is likely to have an important role in the normal physiological response to exercise. Despite accumulating evidence of exhaled NO during exercise, the effects and relevance of NO to exercise are not yet completely understood. Scientific debate surrounds the site of NO production and the stimuli for production. Resolution of these controversial issues will explain the significance of exhaled NO during exercise.