The physiologic phenomenon of alternating congestion and decongestion of the nasal airways was studied by rhinomanometric techniques. This study included the largest reported normal population, 50 persons (32 females, ages 14 to 72 years, mean 25, and 18 males, ages 14 to 50 years, mean 23). The "mask-flowmeter" technique of active posterior rhinomanometry was used to collect data continuously (15-minute intervals) on each subject for about 7 hours. We defined the nasal cycle (in terms of rhinomanometry) as that alternating congestion and decongestion of the nasal airways producing a resistance change (comparing one side to the other, right and left) so that after two consecutive calculations (covering a 30-minute period) the resistance between the sides was at least greater than 20%. By this criterion, we found that 72% of the subjects demonstrated a clearly defined nasal cycle. This resistance difference of 20% between the two sides (right and left) reversed or changed sides at least once for two consecutive calculations during the 7 hours of testing. Normal individuals are not usually aware of this phenomenon because the total nasal resistance usually remains fairly constant and is less than the resistance of either one of the individual nasal passages.