Neutrophil elastase is a mediator common to asthma, chronic obstructive pulmonary disease, and cystic fibrosis and thought to contribute to the pathophysiology of these diseases. Previously, we found that inhaled hyaluronan blocked elastase-induced bronchoconstriction in allergic sheep through its control of tissue kallikrein. Here, we extend those studies by determining if inhaled hyaluronan can protect against the elastase-induced depression in tracheal mucus velocity, a surrogate marker of whole lung mucociliary clearance. We measured tracheal mucus velocity in allergic sheep before, and sequentially for 6 h after, aerosol challenge with porcine pancreatic elastase alone and after pretreatment with 1.5 or 6 mg aerosolized hyaluronan. Elastase (2.55 U) decreased tracheal mucus velocity. Pretreatment with 6 mg, but not 1.5 mg, hyaluronan inhibited the elastase-induced decrease in tracheal mucus velocity. Hyaluronan (6 mg) given 1 h after elastase challenge was ineffective, suggesting the involvement of secondary mediators. The elastase-induced depression in mucus transport appeared to be mediated, in part, by reactive oxygen species and bradykinin because pretreatment with either aerosolized catalase (38 mg/3 ml) or the bradykinin B2-receptor antagonist HOE140 (400 nM/kg) was also effective in blocking the response. These latter two findings are consistent with oxygen radical-induced degradation of hyaluronan with concomitant loss of its regulatory effect on tissue kallikrein, resulting in kinin generation. This hypothesis is supported by the demonstration that hyaluronan failed to block the oxygen radical-induced fall in tracheal mucus velocity resulting from xanthine-xanthine oxidase challenge and that inhaled bradykinin itself can slow mucociliary transport.