Simulations using a biochemically-based model of leaf photosynthesis make it possible to predict the distribution of leaf nitrogen contents that maximizes photosynthetic carbon gain over the canopy of an entire plant. In general, the optimal nitrogen content increased with increasing daily photosynthetically active photon irradiance. Leaf aging in natural environments tended to produce leaf nitrogen contents that were similar to the optimal values but somehwat more clustered. Nitrogen redistribution over the duration of a leaf involves costs that are smaller than the benefits in increased photosynthesis. The costs could become larger than the benefits if nitrogen were redistributed on a shorter time scale.