Technical Abstract: Nitrogen is one of the major variables affecting the response of crop yields to elevated carbon dioxide. Elevated carbon dioxide increases root size, but there are no consistent reports of carbon dioxide effects on nitrogen uptake rates per unit of root. We proposed a simple concept for analysing treatment effects on the nitrogen uptake of plants. Total nitrogen uptake, determined by the rate of depletion of nitrogen from the nutrient solution, was divided into two factors: (1) photosynthesis-driven or "active" uptake, and (2) transpiration-driven or "passive" uptake. Passive uptake was calculated from the amount of nutrient solution transpired and the average concentration of nitrogen in the nutrient solution during the observation, and active uptake was calculated from the total minus the passive uptake. In a short-term experiment, we exposed rice plants to three levels of carbon dioxide for a day. Higher carbon dioxide increased leaf photosynthesis and decreased leaf stomatal conductance. While active nitrogen uptake was slightly increased under higher carbon dioxide, passive uptake was substantially decreased. Consequently, total nitrogen uptake was decreased with carbon dioxide, and there was a close correlation between total uptake and plant transpiration rate. In a long-term experiment, we grew rice at ambient and elevated carbon dioxide from sowing to maturity. Elevated carbon dioxide increased leaf photosynthesis throughout the season. Shoot and root dry weights were higher in elevated carbon dioxide. Active nitrogen uptake sharply decreased with age, but was not much affected by carbon dioxide. Passive nitrogen uptake was lower in elevated carbon dioxide. Again, there was a close correlation between total nitrogen uptake and plant transpiration rate. Our results suggest that the decreased transpiration at elevated carbon dioxide is a key factor reducing nitrogen uptake.