Formaldehyde is a potent sensory irritant in man and animals due to its high solubility in nasal mucosa. Inhalation exposure of formaldehyde has been known to cause degenerative, hyperplastic and metaplastic changes acutely and neoplastic transformation chronically in the respiratory epithelium. The purpose of this study is to investigate the formaldehyde cytotoxicity in the olfactory neuroepithelium and to evaluate the mitotic activity of stem cells for regeneration of olfactory receptor cells in the rat using lectin histochemistry, 5'-bromodeoxyuridine(BrdU) immunohistochemistry and double immunostaining technique of both. The results were as follows : 1) In experimental groups the olfactory epithelium showed degenerative changes such as atrophy and squamous metaplasia, which were observed until two weeks after formaldehyde inhalation. 2) The olfactory epithelium started to recover in three weeks and showed similar state compared to control group in four weeks after inhalation. 3) The number of BrdU labeled cells in the olfactory epithelium was 14.83±1.21/mm in the control group. The mitotic activities were decreased to 4.8±0.8/mm in two weeks and recovered in three weeks after inhalation. 4) The double labeling immunostaining method was performed with proper basal cell specific lectins(GS-I or PHA-L) and BrdU to find the stem cells of olfactory receptor cells. In BrdU labelled cells containing positive reactions to these specific lectins, the proper basal cells occupied 18% in control group and 60.7% in lesioned groups(four days-two weeks after inhalation) and 44.5% in recovery group(three-six weeks after inhalation). These results suggest that formaldehyde gas inhalation cause atrophy and squamous metaplastic changes of olfactory epithelium, and proper basal cells take charge of more active mitotic activity in the regeneration of the olfactory epithelium than globose basal cells after cytotoxic damage induced by formaldehyde gas.