Background: Formaldehyde is a chemical, which is used traditionally for fixing the cadaver. It is vaporized during dissection and practical studying on cadaver. Studies show that this vapour can cause clinical symptoms such as throat, eye, skin and nasal irritation and tears.This study was designed to determine the histopathological changes of rat's tracheal mucous membrane that were exposed to formaldehyde for 18 weeks.Methods: The study was performed on 28, 6-7 weeks postnatal albino Wister rats. The rats were divided into 3 case groups (E1: 4h/d, 4d/w; E2: 2h/d, 4d/w; E3: 2h/d, 2d/w) and 1 control group. The tracheal specimens were sectioned and stained with H&E technique for histopathological study.Results: An epithelial disorganization, cilia disappearance, slight displastic changes and slight subepithelial lymphocytic infiltration were observed in the case of E1. Epithelial disorganization, irregular cilia and slight subepithelial infiltration were seen in E2 and E3 groups.Conclusion: The results of this study show that "the more exposure to formaldelhyde vapour, the more intense epithelial changes".