Nitrogen is taken up by the plant from the soil as nitrate (NO3-) and ammonium (NH4+) and reduced inside the plant to yield amino-N. Accumulation of amino-N in the beet is undesirable because it lowers the technical quality during the process of sugar crystallisation. The aim of the present study was to evaluate the effect of the plant´s N source on content and pattern of amino acids in the beet. The investigations were done on two genotypes which, in prior field trials, showed a different amino-N content of the beet (low- and high-N line). Seeds of these plants were germinated and the plants were grown hydroponically with either 5 mM NO3- or NH4+ as N source in the growth chamber for 40 days. The results show that content and pattern of amino acids of the beet depend strongly on the plant´s N source. Genotypic differences between the two breeding lines are considerably reduced when NH4+ is supplied instead of NO3-. NH4+ as N source lowers the technical quality of the beet.