Reactive oxygen species have been shown in several systems to have a role in tumor promotion and carcinogenesis, although the exact mechanisms are yet to be elucidated. To investigate this further, the effect of hydrogen peroxide was studied in a hamster tracheal organ explant epithelial model. Hydrogen peroxide was added exogenously to tracheal explants maintained in defined serum free media at concentrations of 50, 100, 500 or 1000 microM daily for 1 h for a period of two weeks. The explants were then examined using scanning electron microscopy for evidence of morphologic alteration and for the development of squamous metaplasia. Control tracheal explants maintained in serum free media exhibited normal morphology (except for some decrease in the number of ciliated cells) and developed minimal squamous metaplasia after four weeks in culture. At concentrations of 500 and 1000 microM, hydrogen peroxide was toxic to the epithelium, resulting in complete necrosis of the epithelium within seven days. At concentrations of 50 and 100 microM, hydrogen peroxide treatment resulted within three weeks in the development of a significant degree of squamous metaplasia (covering 52 and 48.7%, respectively, of the surface epithelium). This effect could be negated by the exogenous addition of catalase. This model should be useful in the study of the early cellular events following oxidant injury that contribute to the development of squamous metaplasia.