the concentration of nitrogen which allows a crop to reach its maximum growth rate is called critical concentration. this concentration varies according to the aerial biomass considered, as when this increases, there is a dilution effect of the nitrogen in the plant. the aim of this work was to characterize the nutritional state of foliar nitrogen in two sunflower hybrids (helianthus annuus l.), at different phenological phases, using the nitrogen dilution curve in total aerial dry matter. the treatment was applied to two hybrids, guarani dc and dksol 4040, sown in four furrows by plot, making three replicates per each hybrid. the sampling was made taking three plants per treatment for each replicate, and in four phenological phases: v6; v10; r1; r5.5 and r6. dry matter, nitrogen content and foliar nitrogen were determined by the semi-microkjeldahl method (b). by using the nitrogen dilution curve it could be detected that the content of nitrogen in both hybrids was higher than the required quantity for the maximum growth rate till the flowering, and that the nitrogen values obtained in r1 and r6 were under the curve of dilution of equation (a), indicating a deficit of nitrogen. this methodology is not practical to judge the nitrogenous state of the crop in the stages after flowering, because the composition of the dry matter changes after this phase and the oil synthesis phase begins.