Although clinical reports have described asthma in subjects occupationally exposed to styrene (Moscato et al. 1987; Hayes et al., 1991) the ability of this agent to induce toxic and irritating effects in the respiratory tract has not been investigated in detail.This study describes morphological and biochemical changes occurring in the respiratory system of rats after either inhalation or systemic (i.p.) styrene exposure.In animals exposed for 2 weeks to styrene vapours (300 ppm, 6 hr/day, 5 days/wk), electron microscopy analysis of tracheobronchial mucosa and pulmonary parenchyma showed diffuse cell damage involving the tracheal, bronchiolar and alveolar epithelium. All cell types in the tracheal epithelium were affected. Ciliated cells presented vacuolation, detachment of cilia and blebbing of the apical cytoplasm, or formation of compound cilia. Most secretory cells showed scant secretory granules and blebbings of the apical cytoplasm. Dense bodies and/or fibrillary inclusions were seen in occasional intermediate and basal cells. Styrene inhalation also caused changes in cytoplasmic components of type II pneumocytes and bronchial cells as well as thickness of the alveolar wall. These abnormalities were accompanied by depletion (40%) of GSH content in the lung tissue. GSH was similarly depleted in the liver in the absence, however, of remarkable morphological changes. Injection of single doses of styrene (40 or 400 mg/kg i.p.) induced alterations in the respiratory tract resembling those seen after inhalation exposure but histological damage was more pronounced.Depletion of GSH contents may have a role in pneumotoxic effects as a factor enhancing the vulnerability of the pluripotent type II pneumocytes to styrene-induced cell injury.