Eighteen accessions of Arabidopsis thaliana were grown with low (N-) and high (N+) nitrogen supply. N uptake was monitored by feeding plants with 15N-enriched nutritive solution over 24 h. Biomass [fresh matter (FM) and dry matter (DM)], N concentration (N%), and 15N content were monitored and computed to determine the nitrogen use efficiency (NUE) and nitrogen uptake efficiency (NupE). NUE has been estimated as the ratio between biomass and N concentration (DM/N%) and NupE as the concentration of 15N in plants [microg (g(-1) DM)]. Accession traits were analysed to detect common and individual genotype features. The genetic variation in NUE at high N input was mainly explained by variation in N uptake. Even though plants managed N uptake and N metabolism differently under N+ and N-, NUE was similar in these two conditions, showing that NUE was exclusively genetically determined. Hierarchical classification revealed that the physiological classes arising were similar under N- and N+. Both wasteful and efficient genotypes were detected. Three extreme genotypes, Col-0, Bur-0, and Tsu-0, were noted. Bur-0 and Tsu-0 exhibited high NUE and large biomass. Col-0 showed the reverse: low NUE and low biomass. Bur-0 appeared poorly tolerant of a high N supply. The present data will facilitate the choice of Arabidopsis accessions as parents of recombinant inbred line populations suitable for the mapping of quantitiative trait loci related to NUE, NupE, and N storage capacity.