Ortho-chlorobenzylidene malononitrile(CS) is potent lacrimators and sternutators which has been used recently for riot control and military purposes in Korea and the United states for many years. CS is peripheral sensory irritant materials and it interacts locally with receptors on sensory nerves in the skin and respiratory mucosa. If the exposure is continued, severe inflammation of the conjunctivae, irritation of the nose and the throat arise instantenously. Typical signs and symptoms during exposure to CS gas include eye discomfort, severe lacrimation, blepharospasm, sneezing, rhinorrhea, burning sensation in the throat, coughing, dyspnea, constricting sensation in the chest. Therefore, this study was designed to elucidate the morphological change in the respiratory mucosa of rats by transmission electron microscope following acute exposure to Ortho-chlorobenzylidene malononitrile. The results were as follows : 1) In the epithelial cell at 3hour after exposure, several fragmented cilium are seen and basal bodies are not distinguished definitely. In apical cytoplasm, large multivesicular body and irregular autophagic vacuoles are seen, and free ribosomes are decreased in number. 2) In the epithelial cell at 24hour after exposure, the cilia and microvilli are decreased in number. The pinocytotic vesicles are increased. Numerous fragmented rough endoplasmic reticulum(RER) and irregular multivesicular bodies are seen. 3) In the epithelial cell at 48hour after exposure, intact cilium are seen but several cilium have only normal basal body. Many pinocytotic vesicles are seen, and neutrophils are seen in the intercellular space. The rough endoplasmic reticulum(RER) and lipid droplets are appeared at perinuclear cytoplasm. 4) In the epithelial cell at 4 day after exposure, intact cilia and numerous normal microvilli are appeared. Normal mitochondria and few rough endoplasmic reticulum(RER) are distributed in apical cytoplasm of cell. The scattered rough endoplasmic reticulum(RER), free ribosomes, autophagic vacuoles and mitochondrias are appeared normally. The results suggested that the respiratory epithelial cells of nasal mucosa shows a significant degenerative change after acute exposure to Ortho-chlorobenzylidene malononitrile.