The mechanism of action of secretagogic expectorants (ex., ambroxol) has not been clarified. Recently, attention has been directed to the relationship of their action to pulmonary surfactants. In the present study, the fatty acid (FA) composition of phosphatidylcholine (PC), which is the main physiological surfactant, was investigated using sputum, respiratory tract fluid, mucin-like substance, lung washings and lung tissue of rabbits. Effects of ambroxol (20 mg/kg, i.s.) on several parameters such as output volume, FA contents of PC, protein content and viscosity of respiratory tract fluid of rabbits were also investigated. In respiratory tract fluid, lung washings and mucin-like substance of rabbits, saturated FA, especially C16:0, were predominant components of PC; while in sputum and lung tissue of rabbits and respiratory tract fluid of hens, unsaturated FA, especially C18:1 and C18:2, were more predominant components in comparison with those in the above specimens. Ambroxol significantly increased the contents of C16:0, saturated FA and total FA of PC, and it also increased protein content with an increase in the viscosity of respiratory tract fluid. These results suggest that in the respiratory tract fluid of rabbits, PCs are pulmonary surfactants, and the increasing secretion of pulmonary surfactant is likely to be involved in the expectorant action of ambroxol.