Workers exposed to dicumylperoxide at a chemical plant had crusting and increased occurrence of visible superficial mucosal vessels in the anterior part of the nasal septum. To elucidate the pathogenesis of their symptoms we examined rabbits after acute and long-term experimental airway exposure to dicumylperoxide. An increased amount of mucus containing red blood cells were seen after one hour exposure, as were many club-shaped or distorted cilia. Areas lacking cilia but still covered with microvilli and with goblet cells appeared after a week. After one month's exposure scattered islands with metaplastic, non-ciliated cells were observed and many of the ciliated and microvilli-covered cells, not only the cilia per se, appeared distorted. Goblet cells, scarce in the controls, were observed in increased frequency. Blood vessels with thick, non-fenestrated endothelium were observed in increased frequency and remained two months after the exposure. Free radical scavengers seem to reduce the extent of damage supporting our hypothesis of peroxides as damaging agent.