Nitrogen concentrations of C3 species usually decline after prolonged exposure to elevated CO2. This may result from a dependence of nitrate (NO3-) assimilation on photorespiration. To investigate the relationship between these two processes, we assessed NO3- assimilation under modified atmospheres using 15N techniques. Wheat (Triticum aestivum cv. Veery) seedlings at a photosynthetic flux density of 1000 μmol m‑2 sec-1 were exposed to an ambient atmosphere (360 μmol mol-1 CO2; 21% O2), elevated CO2 (720 μmol mol-1 CO2; 21 % O2), or low O2 (360 μmol mol-1 CO2; 2% O2), and provided with nitrogen as 15N-enriched NO3- during a 12 h uptake period from a nutrient solution. Compared to ambient conditions, shoot nitrate assimilation rates decreased by 20% and 30% under elevated CO2 and low O2, respectively. In an alternative approach, we analyzed the d15N signature of NO3- and total N of wheat seedlings exposed to natural abundance levels of 14N and 15N. Given that the enzyme nitrate reductase discriminates strongly against the heavy isotope, the isotopic signature of organic N indicates the extent to which NO3- had become limited. In the shoots, d15N of organic N was lower, and higher for NO3- , in plants grown under elevated CO2 compared to ambient conditions, indicating that NO3- availability at the site of reduction was more limited under ambient than elevated CO2 concentrations. Both methods thus show that conditions diminishing photorespiration also suppress NO3- assimilation.