The effect of N-acetylcysteine on mucus trasnport velocity (MV), ciliary beat frequency (CBF), mucus production (MP), mucus lysis and on the micro-morphology of the secretory cells was studied in mammalian airways. The results showed that: 1. MV increased in healthy rats and rabbits, as well as in bronchitic rats, after concentrations as low as 10 (-14) g/ml. Depression of MV occurred first at 10 (-6) and 10 (-5) g/ml in healthy and bronchitic animals, respectively. 2. CBF was stimulated at concentrations between 10 (-12) and 10 (-10) g/ml and decreased at concentrations above 10 (-8) g/ml. 3. MP increased by approximately 100% over control values. 4. Lysis of stagnant mucus was evident first at a concentration of 10 (-11) g/ml after 15 min incubation. 5. TEM confirmed the increased activity of the mucus secreting cells and showed that no pathological changes occurred within the cell following incubation at 10 (-7) g/ml for up to 150 min. The importance of these findings on the overall mucociliary function is discussed.