For achieving yield potential, soybeans must maintain high seed nitrogen (N) concentrations and sustain high photosynthesis rates. N in leaves is related to Rubisco, so N fertilization has been proposed to increase the photosynthetic capacity of the leaves. The objective of our work was to evaluate how variations in N supply during grain filling affect respiration and maximum photosynthesis in high yielding soybeans. Three different ways of supplying N were evaluated: No N fertilization (N1), 180 kg ha-1 as polymer-coated urea placed 20 cm below the surface before planting (N2); 180 kg N ha-1 as nitrate ammonium on the surface at R5 (N3). Light responses curves were determined using an LI-6400 at four days between R5 and R6.9 stages. Light response curves were fitted using a non rectangular hyperbola for deriving four parameters: respiration (R), maximum photosynthesis (Pmax), quantum efficiency (��) and physical resistance to CO2 diffusion. �nQuantum efficiency and resistance to CO2 diffusion tended to decrease with time and showed no variations with N treatments. On the other hand, Pmax decreased exponentially with time from 38 ��mol CO2 m-2s-1 2 days after R5 to 13.6 at R6.9. N treatments (N2 and N3) increased Pmax from 12 to 25% during the grain filling period relative to N1. However the decay rate of this parameter during this period was 40% greater in the N fertilized treatments. Pmax increased linearly with N concentration in the leaf up to levels of 5 g N kg-1dry matter. Likewise, R diminished with time from 4 ��mol CO2 m-2s-1 2 days after R5 to 2 at R6.9. N treatments did not affect R during the grain filling period, but at R6.9 N treatments increased R by 50%. A positive relationship was found between N concentration in leaves and R, suggesting higher respiration of N enriched tissue.