Nitric oxide (NO) is a naturally ocurring gas in the human airway.  Its effects are thought to include vasodilation, stimulation of mucociliary function, and microbiostasis, among others.  Nasal NO can be measured either by the Series Method (aspirating gas from one hemi-nasal cavity to the other with closed velopharynx) or by the Parallel Method (exhaling lower airway gas through the two hemi-nasal cavities in parallel).  The role of NO measurement in monitoring the status of upper airway inflammatory diseases (rhinitis and sinusitis) has yet to be defined.