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 (ƒÑ) and physical resistance to CO2 diffusion. ƒnQuantum 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 ƒÝmol 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 ƒÝmol 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.