A linear increase in nitrogen harvest index (NHI), especially when expressed on the basis of cumulative thermal units, has been observed to be a common feature during seed growth in field pea (Pisum sativum L.). The linear increase in NHI was observed even though the individual variables defining plant N uptake and partitioning within the plant changed during seed-fill and were highly variable across a range of growing conditions. A simulation analysis was undertaken in an effort to reconcile the observed linearity in NHI increase when there is variability in the input variables. A wide range of conditions was used including low plant density, water deficit, high-temperature stress and N soil deficiency that resulted in final amounts of accumulated nitrogen ranging from 2.2 to 26.5gm - 2 . The simulations confirmed the linearity in NHI increase, although the values of the NHI increase varied considerably among tests as observed in the experimental results. The explanation for the observed linearity of NHI increase resides in the dominance of plant N uptake over seed N accumulation combined with N transfer from the vegetative tissue to the seed during the early stages of seed growth. Simulations revealed that a reduction in rates of increase of NHI between early and late stages of seed growth had little impact on the overall linear behaviour of NHI increase for the entire period of seed-fill. The analysis also highlighted the importance of total plant N accumulation in influencing grain yield in field pea, especially with regard to the stability of final NHI with an average value of 0.79+/-0.02 (p<0.05).