Longleaf pine seedlings were exposed to two concentrations of atmospheric CO₂ (365 or 720 umol CO₂mol^-1), and two N fertility (40 or 400 Kg N ha^-1 yr^-1) and water treatments (target values of -0.5 or -1.5 MPa xylem pressure potential) in open top chambers from March 1993 through November 1994. Water treatments were imposed after seedling establishment (i.e., 19 weeks after planting). Seedlings were harvested at 4, 8, 12, and 20 months. High CO₂ increased biomass production only when soil was high and relative growth enhancement was greater for the root system. Elevated CO₂ increased root biomass only in water-stressed trees at the final harvest. The proportion of plant biomass in roots and root: shoot ratio only at the final harvest. Adequately-watered seedlings consistently grew larger than water-stressed seedlings only when N fertility was high. We conclude that available soil N was the controlling resource in terms of growth response to elevated CO₂ in this study . Although some growth enhancement was observed for water-stressed trees art increased CO₂ this response was also contingent on available soil N.