To study the effects of nitrogen dioxide (NO2) inhalation on lung lavage surfactant components as markers of an altered surfactant metabolism in type II pneumocytes, rats were exposed to atmospheres with increasing NO2 concentrations (0.8, 5.0, and 10.0 ppm) over 1 and 3 days. After exposure lung lavage was performed and surfactant components as well as lavageable cells analyzed. An increased number of total lavage cells was found with increasing concentration and duration of NO2 exposure. Cell distribution showed an elevation in the number of granulocytes and lymphocytes whereas the number of macrophages was diminished. The amount of total lavage protein revealed an increase related to NO2 concentration and duration. Also the content of lavage phospholipid was increased, with a decreased portion of phosphatidylcholine (PC). Further analyses of PC showed a diminished composition of saturated fatty acids but an elevated content of the unsaturated portion. Functional studies on surfactant phospholipid extracts exhibited comparable values for the surface tension at equilibrium, as well as for the maximal and minimal surface tension of animals exposed to 0.8 ppm NO2 and controls. Higher NO2 concentrations (5 and 10 ppm) resulted in increased values for surface tension compared to controls. This was also observed with purified surfactant that was obtained from controls and from NO2-exposed rats. These experiments show that in vitro exposure of purified surfactant to NO2 atmospheres was more effective than exposure in vivo. When the structure of the surfactant proteins A was studied it was found not to be altered by the NO2. The data clearly demonstrate that NO2 inhalation impaired function of surfactant components that may be used as markers of altered surfactant metabolism.