Effects of N supply, water stress and the water and nitrogen interaction on assimilate partitioning in Populus 'Balsam Spire' hybrid (Populus balsamefera var. Muchuxii (Henery) X Populus trichocarpa var. Hastata (Dode) Farwell.) were studied through three greenhouse experiments. The seedlings in the first experiment received either 1 or 3 or 9 mol N m^-3 over almost a whole growing season. Growth of the different parts of trees increased markedly by N supply. N supply increased whole tree photosynthesis through increasing total leaf area and decreased photosynthetic nitrogen use efficiency. Total respiration also increased with increasing N supply as a result of increasing leaf respiration and below-ground respiration. Below-ground respiration as a proportion of total respiration, however, decreased with increasing N supply. Nitrogen and carbon content and concentrations in different tree components and in the whole tree increased with increasing N supply. Partitioning of nitrogen into different parts of tree, however, showed no response to N supply. The proportion of carbon allocated to the leaves increased while that allocated to the roots decreased as a result of increasing N supply through increasing the concentrations of structural carbon compounds at the expense of those of non-structural carbon compounds. The ratio of total non-structural to total structural carbon compounds, therefore, increased with increasing N supply. In the second experiment, three treatments were applied to trees in the second half of their growing season by withholding irrigation either suddenly or gradually.