Air pollution studies have shown that nitric oxide (NO), a gaseous free radical, is a potent photosynthetic inhibitor that reduces CO ₂ uptake activity in leaves. It is now recognized that NO is not only an air pollutant but also an endogenously produced metabolite, which may play a role in regulating plant cell functions. Although many studies have suggested the presence of mammalian–type NO synthase (NOS) in plants, the source of NO is still not clear. There has been a number of studies indicating that plant cells possess a nitrite–dependent NO production pathway which can be distinguished from the NOS–mediated reaction. Nitrate reductase (NR) has been recently found to be capable of producing NO through oneelectron reduction of nitrite using NAD(P)H as an electron donor. This review focuses on current understanding of the mechanism for the nitrite–dependent NO production in plants. Impacts of NO produced by NR on photosynthesis are discussed in association with photo–oxidative stress in leaves.