The effect of administration of a single therapeutic dose of Intussin on the ultrastructure of the rabbit tracheal epithelium was investigated. Twenty minutes after the application of five drops of Intussin only slight reaction of the pseudostratified ciliated epithelium was recorded. The ciliated cells manifested only slight signs of pathological alteration. Stimulation of 36 +/- 2% of goblet cells and degeneration of 7 +/- 2% of these cells were recorded. The average number of kinocilia per 1 microns2 of the ciliary border area was significantly (P less than 0.005) reduced to 6.7 +/- 0.5. However 97 +/- 2.7% of kinocilia remained intact. There was slight but significant (P less than 0.005) increase in number of pathological and degenerated kinocilia. As morphological signs of impaired mucus flow clumps and layers of inspissated secretion and numerous bacteria were found in the area of the ciliary border. Owing to the slight damage to the ciliary border and to the low degree of stimulation of the goblet cells the impairment of the self cleaning ability of the epitselium was obviously due to the ciliostatic action of some component of this preparation.