While N supply has a large effect on growth and productivity of field-grown maize, its impact on leaf and canopy photosynthesis is less clear. Our objective was to characterize how N supply, and use of N by the maize plant impacts photosynthesis under field conditions. The N supply/N use was altered by fertilizing with a deficient to an adequate rate of N (0, 56, and 224 kg N ha^-1), and by selecting two hybrids with different N use strategies (a standard commercial and a high-protein experimental hybrid). Photosynthetic measurements were taken between tassel emergence (VT) and the grain milk stage (R3), while total biomass and N accumulations were also determined at physiological maturity (R6). Both hybrids exhibited the typical patterns of biomass and N accumulation with the biggest difference in plant N occurring at the highest rate. Plant height and leaf area index (LAI) were both increased by N-supply (qualitative for height, and linear for LAI), especially for the high protein hybrid. For both hybrids, N-induced differences in leaf carbon exchange rate (CER), and the activities of PEP carboxylase and Rubisco, were already evident at VT and strongly apparent at R3. CER, and Rubisco activity decreased between VT and R3, and this decrease was ameliorated with N supply. For the most part, the N-induced changes in leaf photosynthesis reflected the seasonal differences in biomass accumulation. Our data suggests that N-induced changes in photosynthesis are a result of an improvement in photosynthetic area and activity during vegetative growth, and a delay in the dismantling of the photosynthetic machinery during grain fill.