Abstract:The effect of increasing doses of mineral nitrogen fertilization (40, 80, 120 and 160 mg N kg -1 soil) on chemical composition of lucerne (Medicago sativa L.) was studied in pot trial at the Institute of Forage Crops, Pleven, Bulgaria. The plants were grown under optimum water supply (75-80% FC) and 10-days water deficiency stress (37-40% FC) at the stage of budding of lucerne. It was found that with increasing doses of mineral nitrogen fertilization, crude protein content in dry aboveground mass increased from 13 to 19% under the conditions of optimum moisture, and decreased from 6 to 14% under water deficiency stress. Crude fiber content in root mass under water deficiency stress increased most in unprovided and provided plants with low dose of mineral nitrogen. Ca:P ratio in dry aboveground mass remained almost unchanged in the conditions of optimal moisture, and increased significantly in the conditions of water-deficiency stress.