The possibility of improving nitrogen use efficiency in maize was investigated using S1 endogamic families of the populations Sol da Manhã NF and Catetão. A simple 10 × 10 lattice design was adopted and the trials carried out at the experimental field of MITLA AGRÍCOLA LTDA, in Uberlândia, State of Minas Gerais, during the 1994/95 planting season. Based on grain production figures, the three best and three worst performing S1 endogamic families were selected for this study. These were pooled to form four sub-populations denominated NFB, NFR (the best and worst families, respectively, of the Sol da Manhã NF variety), CATB and CATR (the best and worst families, respectively, of the Catetão variety). Each of these sub-populations was evaluated under greenhouse conditions. The experimental design was factorial with treatments arranged in randomized blocks. Sample replicates consisted of pots with four plants. Feeding with modified Hoagland's nutrient solution began on the seventh day after sowing. The study involved four nitrogen regimes, where varying proportions of NO3 - and NH4 + were formulated, such that the nutrient solution contained the following mixtures: 75% NO3 -: 25% NH4 +; 25% NO3 -: 75% NH4 +; 50% NO3 -: 50% NH4 + (all high N mixtures) and 5% NO3 -: 5% NH4 + (low N mixture). Twenty-five days after planting, the activities of the enzymes nitrate reductase and glutamine synthetase (transferase and synthetase assays) were determined for the leaves using the third topmost expanded leaf of the four plants in each pot. 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