Previous research indicated genotype-specific responses in nitrogen utilization efficiency (NUE) for oilseed rape (Brassica napus L.), but mechanisms behind those differences arc unknown. Our objective was to determine whether cultivar variations in NUE (dry matter production per unit of N absorbed) could be related to the differences in N and nitrate uptake and partitioning. Four Australian spring canola cultivars (Eyre, Charlton, Pinnacle and Rainbow) were grown in glasshouse under conditions of low- and high-N supply. All cultivars were at the same growth stage when harvested 60 days after sowing (rosette stage with five leaves at low-N supply and nine leaves at high-N supply). Cultivars significantly differed in total N uptake in roots and shoots, but not when the whole plants were considered. Although all cultivars had similar total N uptake per plant, significant differences in NUE existed because of differences in plant biomass. N-efficient cultivars Charlton and Rainbow produced larger plant biomass and had lower N concentration in various aboveground plant parts (including dead leaves) than N-inefficient cultivars Pinnacle and Eyre. Nitrogen concentration in roots did not differ significantly among cultivars. Regardless of N supply, N concentrations in various plant parts were in the order: young blades > old blades > roots > young petioles > stems > old petioles > dead leaves. No significant variation in nitrate-N concentration in roots or various aboveground plant parts was found among tested cultivars. The absence of cultivar x N treatment interaction for plant dry weight, N concentration, N uptake, and consequently NUE, clearly indicated that cultivars that performed best at high-N supply also showed similar responses under N-deficient conditions. Despite similar total N uptake per plant, significant differences in NUE existed because more N-efficient cultivars produced larger plant biomass and tended to have lower N concentrations in all plant parts (except roots) compared with less N-efficient cultivars. (c) 2005 Published by Elsevier B.V.