To define the site of release and factors modulating exhaled nitric oxide (NO) during exercise in humans, we measured exhaled NO output during exercise, during exercise after balloon occlusion of the nasopharynx (to exclude nasal NO), and at rest with isocapneic hyperventilation or dobutamine infusion. Exhaled NO output increased from rest to exercise (57 +/- 10 to 171 +/- 30 nl.min-1.m-2; P < 0.003; n = 8). Exclusion of nasal NO reduced exhaled NO at rest and during exercise. Calculated nasal contribution at rest (53 +/- 5%) decreased during exercise (29 +/- 6%; P < 0.05), whereas nonnasal contribution increased (47 +/- 5 to 71 +/- 6%; P < 0.05). Isocapneic hyperventilation at rest increased exhaled NO output (51 +/- 8 to 94 +/- 22 nl.min-1.m-2; P = 0.05). Dobutamine infusion did not increase exhaled NO output. We conclude that nasal exhaled NO decreases (and nonnasal exhaled NO increases) with exercise. We also conclude that, under the conditions of this study, increased exhaled NO output during exercise is more closely related to increased ventilation than to increased blood flow.