Published as Paper No. 9750, Journal Series, Nebraska Agricultural Research Division.Reactions involving changes that affect the function of leghemoglobin (Lb) are reviewed. The chemical nature of Lb and conditions inside nodules, such as slightly acid pH and the presence of metal ions, chelators, and toxic metabolites (nitrite, superoxide radical, peroxides), are conducive for oxidation of ferrous Lb (Lb2+) or its oxygenated form (LbO2) to nonfunctional ferric Lb (Lb3+) and ferryl Lb. Because Lb3+ is nearly nonexistent in nodules and undergoes observable reduction in vivo, mechanisms must operate in nodules to maintain Lb in the Lb2+ state. Redox reactions of Lb are mediated, for the most part, by activated oxygen species: (a) oxidation of LbO2 to Lb3+ involves superoxide; (b) excess peroxide oxidizes LbO2 and Lb3+ to ferryl Lb and may cause breakdown of heme, release of iron, and generation of hydroxyl radicals (protein radicals may be formed in this process); (c) enzymatic reduction of Lb3+ requires active flavin and thiol groups and involves formation of peroxide; and (d) direct reduction of Lb3+ by NADH is mediated by superoxide and peroxide. Transition metal ions and certain small molecules of nodules such as flavins may act as intermediate electron carriers between NADH and Lb3+, increasing the rate of reaction, which then proceeds via superoxide or flavin radicals, respectively.The original work reported here received financial support from the NRI Competitive Grants Program/U.S. Department of Agriculture (grant No. 91-37305-6705) to R.V.K., and from the Consejo Asesor de Investigación-Diputación General de Aragón (grant No. PCB-10/90) to M.B.Peer reviewed