The effect of short-term nitrate application (10 mM, 0-4 d) on nitrogenase (N2ase) activity, antioxidant defenses, and related parameters was investigated in pea (Pisum sativum L. cv Frilene) nodules. The response of nodules to nitrate comprised two stages. In the first stage (0-2 d), there were major decreases in N2ase activity and N2ase-linked respiration and concomitant increases in carbon cost of N2ase and oxygen diffusion resistance of nodules. There was no apparent oxidative damage, and the decline in N2ase activity was, to a certain extent, reversible. The second stage (>2 d) was typical of a senescent, essentially irreversible process. It was characterized by moderate increases in oxidized proteins and catalytic Fe and by major decreases in antioxidant enzymes and metabolites. The restriction in oxygen supply to bacteroids may explain the initial decline in N2ase activity. The decrease in antioxidant protection is not involved in this process and is not specifically caused by nitrate, since it also occurs with drought stress. However, comparison of nitrate- and drought-induced senescence shows an important difference: there is no lipid degradation or lipid peroxide accumulation with nitrate, indicating that lipid peroxidation is not necessarily involved in nodule senescence.Supported by grants PB92-0058 and PB95-0091 from the Di- rección General de Investigación Científica y Técnica to M.B. Work at the Institute of Grassland and Environmental Research was funded through the Biotechnology and Biological Sciences Re- search Council. P.R.E. and Y.G. were the recipients, respectively, of a predoctoral fellowship and a postdoctoral contract from the Ministerio de Educación y Ciencia. I.I.O. was the recipient of a predoctoral fellowship from Gobierno Vasco.Peer reviewed