The physiological role of nitric oxide (NO) is being investigated in many experimental and clinical settings. There is considerable evidence that NO is involved in the regulation of lung vascular function. In addition there are many studies reporting the beneficial effect of NO inhalation. NO formed from L-Arginine has been detected in exhaled breath using indirect mass spectrometry and chemiluminescence. Both methods provided good evidence for the presence of NO in breath samples but were not unequivocal. We therefore developed a method using gas-chromatography-mass spectrometry which allowed us to measure trace levels of NO in air and breath. Eight healthy volunteers supplied numerous breath samples for analysis. A clear peak for nitric oxide was observed in seven volunteers. The mean level was 13ppb (n = 7, range < 2 to 19ppb). This data is in good agreement with our previous data and unequivocally confirms the presence of nitric oxide in human breath.