The level of nitric oxide (NO) in exhaled air fluctuates in normal individuals depending on the physiological conditions. We evaluated the effects of duration of exhalation and breath-holding on the exhaled concentrations of NO in 16 normal human volunteers. Exhaled gas corresponding to vital capacity was collected in 6-liter Tedlar bags and analyzed by chemiluminescence. The NO concentration in exhaled gas increased significantly in proportion to the duration of exhalation [P = 0.009 +/- 0.011 (SD)] and was increased after breath-holding. There was no significant difference in the exhaled NO concentration among 10-s phases of a 30-s exhalation, as determined from multiple breath collections. The NO released from the airways is presumably unaffected by fluctuation of exhalation speed. The NO release rate, calculated from a single regression analysis between the NO concentration and the duration of exhalation, was 39 +/- 29 pmol/s, a value which was about fourfold greater in nine patients with bronchial asthma.