A three-year study (1988-89-90) was carried out in the South-West of France in about thirty farms. Its purpose was to evaluate the optimum amounts of nitrogen that filling flue-cured crop has to uptake to achieve a maximum yield without decreasing the quality. Representative samples of crops were analyzed after curing. Results have enabled to calculate the nitrogen amount taken up by the plants. This amount was put in relation with yield and quality of crop. In South-West of France, the best results were obtained when the plant had taken up 80 to 100 kg of nitrogen per hectare. Lower nitrogen amounts are a limiting factor for yield; bigger amount, involve a degradation of quality. In many cases, results are lower than expected in spite of optimum nitrogen supply (90 to 110 kg/ha). Other factors (soil cultivation, irrigation, climate...) may limit the absorption of this element. This work confirms the importance of nitrogen assessment which enables to select the best plots (possibly not to take one) and to reason better the complementary contribution of nitrogen fertilization.