The direct effect of mucolytic agents aerosol on the ultrastructure of the tracheal epithelium of healthy rabbits was studied. Due to the function of mucolytics the ciliated cells are not altered significantly, the goblet ones are overstimulated and injured and after rapid evacuation of mucus they degenerate and are expelled from the epithelium. There are pronounced quantitative differences in the degree of damage caused to the airway epithelium by propylene glycol (10%) plus Broncholysin (20%), 20% Broncholysin, 10% Broncholysin or 4% Mistabron. Most pronounced injury of the epithelium was produced due to propylene glycol-Broncholysin application, 4% Mistabron is the least irritating substance, the next suitable mucolytic solution is 10’% Broncholysi