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The response of plant N relations to the combination of elevated CO₂ (eCO₂) and warming are poorly understood. To study this, tomato (<i>Solanum lycopersicum</i>) plants were grown at 400 or 700 ppm CO₂ and 33/28 or 38/33 °C (day/night), and their soil was labeled with ¹⁵NO₃⁻ or ¹⁵NH₄⁺. Plant dry mass, root N-uptake rate, root-to-shoot net N translocation, whole-plant N assimilation, and root resource availability (%C, %N, total nonstructural carbohydrates) were measured. Relative to eCO₂ or warming alone, eCO₂ + warming decreased growth, NO₃⁻ and NH₄⁺-uptake rates, root-to-shoot net N translocation, and whole-plant N assimilation. Decreased N assimilation with eCO₂ + warming was driven mostly by inhibition of NO₃⁻ assimilation, and was not associated with root resource limitations or damage to N-assimilatory proteins. Previously, we showed in tomato that eCO₂ + warming decreases the concentration of N-uptake and -assimilatory proteins in roots, and dramatically increases leaf angle, which decreases whole-plant light capture and, hence, photosynthesis and growth. Thus, decreases in N uptake and assimilation with eCO₂ + warming in tomato are likely due to reduced plant N demand.