To investigate the effects of sulfur dioxide (SO2) on olfactory epithelium, an experiment was performed with 56 mice from the same colony. Experimental animals were divided into three groups consisting of a 30-min exposure group (group 1), a 60-min exposure group (group 2), and a 120-min exposure group (group 3). The olfactory mucosa in these mice were studied by light microscopy immediately, and after 24 h, 48 h, or 72 h exposure to 20 ppm of SO2. Edema, loss of cilia, epithelial thinning, and epithelial desquamation in the olfactory epithelium were observed in groups 2 and 3. The basal lamina and the connective tissue were well preserved throughout the entire mucosa. Injuries to olfactory epithelium became severer with exposure time. These changes were further pronounced 24 h after exposure. Regenerated epithelia were not observed in any group. Scanning electron microscopic findings were consistent with light microscopic findings. Olfactory epithelial surface were consistent with light microscopic findings. Olfactory epithelial surface was sloughed off and revealed, underlining intact basal lamina. The results of this study suggest that early lesions of olfactory epithelium after exposure to SO2 may be primarily degenerative.