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The Metabolic Nitrate Pool (MNP) was measured in leaves by using the in situ nitrate reductase (NR) activity assay, as described Robin et al. [Physiol. Veg. 1983, 21:155-122]. Total nitrate (NCV) content and in vitro NR activity were also determined as indexes of the NO3- nutrition status of pure hydroponic grown maize plants. Two experiments were designed: i) increasing concentrations of NO3- in the nutrient solution, and ii) deprivation of NO3- in the nutrient solution. Although a time course accumulation of nitrite (NC2-) occured after the incubation of maize leaves in anoxia and darkness in the in situ NR activity assay, the MNP determination was no consistent with the NC3- treatments. The MNP was small in excess of NO3- (30 mM), whereas the total NO3- content and the in vitro NR activity were the highest. Moreover, in the NO3- depletion experiment, the MNP increased after 6 h deprivation, while the total NO3-concentration and the in vitro NR activity decreased. The Metabolic Nitrate Corrected Pool (MNCP) was calculated from the results of the MNP and the in vitro NR activity, under the assumption that part of the NO2- accumulated under anoxia and darkness was reduced by a putative residual nitrite reductase (NiR) activity. MNCP showed a better correlationship with the total content of NO3- in shoot than MNP did in both experiments. Therefore, our results suggest that the MNCP could provide useful information about the physiology of the NO3-assimilation, but it should be cautiously used as a tool for the diagnosis of the NO3- nutrition status of aerial parts of maize plants.