Exhaled nitric oxide (NO) may aid in monitoring pulmonary disease. The single-breath NO profile (subjects with nose clip) was described as a NO peak followed by a plateau (NO(PLAT)). Published exhaled NO values vary greatly, possibly due to contamination with nasal NO and differing respiratory maneuvers. We developed a technique to measure pulmonary NO, without nasal NO, by having the subject maintain a positive expiratory pressure (ensuring vellum closure), and we examined the variation in NO(PLAT) over a range of expiratory flows (4.2 to 1,550 ml/s). NO(PLAT) values rose almost 35-fold (3.2 +/- 1.4 ppb to 110.5 +/- 54.8 ppb) with decreasing flow, described by NO(PLAT) = 208.6795 x (flow rate)(-0.5995). However, NO excretion showed an almost 11-fold rise as flow increased. In summary, we present a simple technique for measuring exhaled NO without contamination by nasal NO. There is a marked flow dependence of exhaled NO concentration and excretion. Exhaled pulmonary NO is best measured at very low flow rates to amplify the signal and must be related to the expiratory flow employed.