p6high correlation between the usage of kerosene and the appearance of asthmatic crises has been demonstrated. The ultrastructure of the upper respiratory tract of animals treated with kerosene has not been previously reported. Kerosene aerosol was administered for 15 minutes daily during 21 days to adult male guinea pigs with fragments of trachea being processed for ultrathin electron microscopical studies. Controls did not receive any treatment. Trachea of guinea pigs submitted to kerosene aerosols showed swelling, ruffling and breakdown of the ciliary membrane. The regularly arranged ciliary border was disturbed to a certain degree in some areas by the development of cytoplasmatic protrusions at the apical part of the ciliated cells. An eosinophilic infiltrate was observed deep inside the epithelium and into the lamina propria. Therefore, these ciliary alterations can be considered as one of the most important changes induced by kerosene in tracheal epithelial cells. The protrusions may represent a sign of cell alteration produced by kerosene aerosol inhalation in the guinea pig.