A study of rats acutely exposed to ozone (0.5--2.0 ppm) or nitrogen dioxide (2--20 ppm) for 2 hr and sacrificed immediately thereafter shows little similarity in the individual biochemical effects of these pollutants. No evidence of nitrogen dioxide-induced lipid peroxidation was observed. Of interest is the finding that inhalation of nitrogen dioxide increases the extent to which concanavalin A agglutinates alveolar macrophages while ozone has exactly the opposite effect.