A field study was conducted in Córdoba, Spain, to assess the effect of N fertilizer on the N content, N accumulation, N partitioning, N remobilization and their relationship with seed yield. Two sunflower hybrids, early (AB-E353) and late (Sungro 380) season types, were grown in sandy loam soil (Typic xerofluvent). Nitrogen at rates of 150 kg ha'1 (1987) and 300 kg ha"1 (1988) was applied before planting. In both years, the N content (%) and N accumulation (g m'2) in the organs of sunflower increased with N fertilization. The highest N concentrations were observed during early growth stages; N concentrations subsequentially decreased until physiological maturity. Nitrogen uptake ceased earlier in N-fertilized sunflower than in unfertilized sunflower and this was reflected in a lower nitrogen harvest index (NHI). In N+ a greater percentage of N accumulated in the seeds was supplied by the vegetative organs. This redistribution increased in relation to the N and genotype growth cycle. The leaf was the organ which accumulated most N and which made the greatest contribution to the seeds. The amount of N supplied by the shoot to the seeds and N accumulation in the leaf (g m'2) 66 days after emergence showed a higher correlation with the seed yield.