Plants are more susceptible to the toxic effects of nitrogen dioxide when exposure takes place in the dark. Beta-carotene and other common carotenoids react with nitrogen dioxide in the dark to yield intermediate nitrosating agents consistent with the formation of nitrate esters. Simultaneous exposure of carotenoids to NO2 and light significantly reduced formation of nitrosating intermediates and resulted in the release of nitric oxide (NO) into the gas phase. Light-mediated reduction of NO2 to NO by carotenoids may be an important mechanism for preventing damage in plants exposed to NO2. The formation of nitrosating agents from the reaction of carotenoids with NO2 suggests that their ability to prevent nirosative damage associated with NO2 exposure in both plants and animals may be limited in the absence of light.