The mechanism of the post-translational modulation of nitrate reductase activity (NR, EC 1.6.6.1) is briefly summarized, and it is shown that by this mechanism nitric oxide production through NR is also rapidly modulated. New and partly unexpected details on the modulation mechanism have been obtained by using immunological techniques. The phosphorylation state of NR has been assessed with peptide antibodies raised against the serine phosphorylation motive of spinach NR. By co-immunoprecipitation experiments, 14-3-3 binding to phospho-NR and the function of Mg(2+) in that process has been elucidated. Conflicting data on the role of NR phosphorylation and 14-3-3 binding in controlling NR proteolysis are discussed. A possible role of other NR inactivating proteins is also briefly considered and the regulation of NR of Ricinus communis is described as an interesting special case that differs from the 'normal' mechanism in several important aspects.