Nitric oxide (NO) is detectable in the exhaled gas of adults during spontaneous respiration and, according to current knowledge, mainly originates from the paranasal sinuses. We studied total NO excretion rates by chemiluminescence in preterm infants whose paranasal sinuses are known to be only partially pneumatized. NO excretion was 7.15 +/- 1.13 nl/min (mean +/- SD, range 6.33-9.36 nl/min) measured from spontaneously exhaled nasal gas (n = 6) and 0.3 +/- 0.05 nl/min (range 0.26-0.36 nl/min) measured from the lower airways in intubated individuals (n = 3). These values are considerably lower than those reported for older children and adults. Body weight-related amounts of NO excretion, however, seem comparable between infants and adults.