In chronic heart failure (CHF), the ventilatory response is increased compared with normal. This response is, in part, caused by reduced perfusion to ventilated lung. Nitric oxide (NO) is a potent vasodilator and may have an important role in pulmonary vasodilatation during exercise. NO is present in exhaled air. The amount of NO in exhaled air, when breathing NO-free compressed air, is known to increase in normal subjects during exercise. In this study, we quantified NO output in exhaled air in patients with CHF during exercise. Six patients with CHF (New York Heart Association Class II and III; two with dilated cardiomyopathy, three with ischemic heart disease, and one with hypertensive heart disease) and six normal subjects were studied with a symptom-limited incremental exercise test on a cycle ergometer. Oxygen uptake (v̇O₂ ), carbon dioxide output (v̇CO₂ ), and minute ventilation (v̇E) were measured breath by breath with a mass spectrometer, flow meter, and computer. The NO concentration was continuously measured in mixed expired air by chemiluminescence. Peak exercise work rate was lower in patients with CHF than in normal subjects (71.3 ± 41.6 W vs 257.0 ± 49.7 W; p < 0.01). Patients with CHF showed a higher v̇E/v̇CO₂ level at peak exercise than normal subjects (CHF, 47.0 ± 10.7; normal subjects, 35.6 ± 5.2; p < 0.01). NO concentration of exhaled air at rest was lower in CHF patients than in normal subjects (4.0 ± 2.2 ppb vs 10.5 ± 6.2 ppb, respectively; p < 0.05). NO output from the respiratory tract (v̇NO) was significantly lower in patients with CHF compared with normal subjects at rest (45.3 ± 24.3 nl/min, 117.5 ± 60.1 nl/min, respectively, p < 0.05), and although it increased during exercise, it did not increase in patients with CHF as much as in normal subjects (75.3 ± 43.4 nl/min vs 512.9 ± 253.6 nl/min, respectively; p < 0.01). The increase above rest (exercise/rest) was smaller in patients with CHF than in normal subjects (2.10 ± 1.92 vs 4.81 ± 2.67, p < 0.05). These data support the concept that the smaller increase in NO production (v̇NO) during exercise may be responsible for a blunted vasodilation in patients with CHF, resulting in a smaller reduction in dead space/tidal volume and v̇E/v̇CO₂ at the lactic acidosis threshold than normal. This finding may play a role in the abnormally high ventilatory response to exercise in patients with CHF.