A major aim in sugar beet breeding is to decrease the concentration of soluble N compounds in the beet to avoid negative impacts during beet processing. Due to its importance and analytical limitations the only selection criterion is amino N analyzed in the beet at harvest. It is thus not clear when genotypic differences are established during the season. Furthermore, it is not known whether selection for low amino N affects other N compounds in the beet and the N composition of other plant organs. Therefore, the concentrations of total N and soluble N compounds (protein, amino N, glutamine, betaine, nitrate) were investigated during the season in different organs (leaf blades, petioles, crown, beet) of two sugar beet genotypes differing in the amino N concentration of the beet. Field trials were carried out at three sites in 2002 and in 2003 with harvests at three times from 100 to 170 days after sowing. Differences between the genotypes in N compounds were already established at 100 days after sowing. Therefore, characterization of genotypic differences seems to be possible earlier than in autumn. In the target organ beet, largest genotypic differences occurred for the target trait amino N and its main constituent glutamine, whereas the concentration of total N, betaine and soluble protein was only slightly lower in the low compared to the high amino N genotype. In the other organs, no consistent changes occurred for the soluble N compounds. The largest genotypic differences were detected in the nitrate concentration of the mature leaf blades (48 % lower) and the glutamine concentration of the young leaf petioles (47 % higher in the low amino N genotype). Glutamine synthetase activity of the beet was positively correlated to its amino N concentration. It was concluded that differences between genotypes in the amino N concentration of the beet resulted most likely from differences in glutamine synthetase activity in the beet but not in the leaves.