The activity of the nitrate reductase (NR) was determined in 5 lettuce cultivars grown in a NFT system during summer conditions. To determine the effects of the quality of the light over the growth and development of the lettuce plants, 50% blue, black and aluminum shadow nets were used. The data were analyzed through a completely random design in a factorial arrangement (light quality × cultivar). The activity of the NR was expressed as μmol of NO2-formed (g fresh weight h-1). The nitrites concentration was between 0.52 and 1.15 μmol. The lowest nitrites concentration was detected in the five cultivars that were grown under the blue net conditions (0.52 to 0.67 μmol), meanwhile, the highest nitrites concentration was found in the environment protected with the aluminum net (0.54 to 1.15 μmol). The lower and higher nitrite concentrations were found in the cultivars 'Crufia' and 'Prize Head' respectively. The higher yield was found in the plants grown under the black net conditions, with the highest weight obtained in the 'Fanfare' cultivar (375.50 g plant-1) and the lower yields were found in plants grown under control conditions (without net). The lettuce plants grown under the blue net were smaller and more compact and showed a retarded blooming period with respect to the other treatments. It was demonstrated that the lettuce culture by hydroponic systems shows a higher activity of the NR. Hydroponics should be a good alternative for lettuce production with low nitrate contents.