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A set of N-fertilization field experiments was used to determine the 'critical nitrogen concentration', i.e, the minimal concentration of total N in shoots that produced the maximum aerial dry matter, at a given time and field situation. A unique 'critical nitrogen dilution curve' was obtained by plotting these concentrations Nct (% DM) vs. accumulated shoot biomass DM (t ha-1). It could be described by the equation: Nct = 5·35DM-0·442 when shoot biomass was between 1·55 and 12 t ha-1. An excellent fit was obtained between model and data (r2 = 0·98, 15 d.f.). A very close relationship was found using reduced N instead of total N, because the nitrate concentrations in shoots corresponding to critical points were small. The critical curve was rather close to those reported by Greenwood et al. (1990) for C3 plants. However, this equation did not apply when shoot biomass was less than 1·55 t ha-1. In this case, the critical N concentration was independent of shoot biomass: the constant critical value Nct = 4·4% is suggested for reduced-N. The model was validated in all the experimental situations, in spite of large differences in growth rate, cultivar, soil and climatic conditions; shoot biomass varying from 0·2 to 14 t ha-1. Plant N concentration was found to vary by a factor of four at a given shoot biomass level. In the heavily fertilized treatments, shoot N concentration could be 60% higher than the critical concentration. Most (on average 80%) of the extra N accumulated was in the form of reduced N. The proportion of nitrate to total N in shoot mainly depended on the crop stage of development. It was independent of the nitrogen nutrition level.Copyright 1994, 1999 Academic Press