In previous studies, we have reported bronchoalveolar lavage (BAL) findings following single and repeated exposures to NO2. Here, we present the first data on measurements of the frequency of the mucociliary activity in healthy subjects following exposure to NO2. Twenty four healthy nonsmoking volunteers underwent fibreoptic bronchoscopy, during which mucociliary activity was measured using an in vivo method based on a two-way fibreoptic system, to obtain baseline data. Two weeks later, the subjects were reinvestigated following NO2 exposure. Eight volunteers were investigated 45 min after a 20 min exposure to 1.5 ppm NO2 (Group A); eight subjects 45 min after a 20 min exposure to 3.5 ppm NO2 (Group B); and eight subjects 24 h after a 4 h exposure to 3.5 ppm NO2 (Group C). No activity was detected in Group A or B 45 min after exposure, in contrast to the easily identifiable mucociliary activity waves at the reference bronchoscopy. In group C, a significant elevation of the frequency of the mucociliary activity waves was seen at 24 h. It is concluded that short-term exposure to NO2 in man produces a significant reduction in the mucociliary activity 45 min after exposure. The effect ceases within 24 h. Further studies are needed to evaluate the effects of repeated exposures with NO2 on mucociliary activity in man.