The interaction between nitrogen application rate and carbon assimilation in potato strongly affects growth and development processes. Two consecutive experiments were carried out in the summer of 2005 in six sunlit, controlled environment plant growth chambers with six nitrogen application rates and two levels of CO2, 740 μmol mol-1and 370 μmol mol-1. The six nitrogen treatments were 2, 4, 6, 8, 11 and 14 umol L-1 of N. Leaf expansion rates of selected leaves was measured twice weekly, whole plant canopy photosynthesis, light interception, and transpiration were measured at 5 and 15 minute intervals. At the end of the season the plants were separated into leaves and stems of individual components, and measured for area, weight, carbon, and nitrogen contents. Elevated CO2 increased tuber weight, but not above ground biomass in all N treatments. The nitrogen contents in all N treatments were lower at elevated CO2 than at ambient and the carbon content higher. Higher nitrogen applications increased the rate of secondary and tertiary branch growth in both CO2 treatments. Elevated CO2 appeared to enhance branching, especially at higher N levels. Nitrogen uptake and effects on partitioning appear to be related to carbon assimilation rates and the C/N ratio is an important factor.