Large interindividual variations of nasal mucocilliary clearance across healthy subjects have been previously described. The aim of this study was to evaluate the respective role of the nasal mucus quality and ciliary activity in determining nasal mucociliary clearance and to study changes in these variables across time and with environmental conditions. In 20 healthy nonsmoking volunteers, the in vivo nasal mucociliary clearance was measured with the saccharin test. Then, nasal mucus and ciliated cells were collected. The in vitro ciliary beat frequency of the nasal cells was evaluated by a photometric analysis. The in vitro nasal mucus transport rate was evaluated by use of the frog depleted-palate model. The nasal mucociliary transport time (NMTT) of saccharin was greater than 30 min in seven subjects and was 13.6 +/- 6.1 min (mean +/- SD) in the remaining 13 subjects. NMTT was correlated to the in vitro transport rate of the nasal mucus (r = -0.75, p less than 0.001), but not to the ciliary beating frequency or to ambient temperature, relative humidity, or air pollution indices measured. Moreover, large intraindividual variations of NMTT, measured on two occasions 4 to 8 wk apart, were found to be significantly correlated with changes in mucus transport rate (r = -0.60, p less than 0.05).