An experimental model was designed to characterize lesions in the lung of lambs inoculated with bovine respiratory syncytial virus (BRSV). Twenty-five Merino lambs of both sexes, with a live weight of 17 +/- 3 kg, received an intratracheal inoculation of 20 ml saline solution containing 1.26 x 10(6) TCID50 BRSV (strain NMK-7) per ml. Lambs were slaughtered 1, 3, 7, 11 and 15 days postinoculation (DPI), and samples were taken for analysis using light-microscopic and immunohistochemical techniques. The results reflected the effect of the virus on airway epithelia. The presence of BRSV in ciliated bronchial and bronchiolar epithelial cells gave rise to cytopathological changes, including loss of cilia and cell necrosis; these changes might be expected to decrease the efficiency of mucociliary clearance, favouring the development of secondary bacterial bronchopneumonia. These results suggest a reduction in BRSV tropism for alveolar epithelia compared to bronchial and bronchiolar epithelia. Light-microscopic analysis revealed a narrowing of alveolar and airway lumina and a considerable interstitial inflammatory reaction.