In this study the impact of an elevated CO2 level on allocation of assimilates and water use efficiency of Douglas fir [Pseudotsuga menziesii (Mirb.) Franco] was investigated. Juvenile Douglas firs were exposed to a long-term treatment at 350 and 700 μ 1-1 CO2 for 14 months and subsequently crosswise transferred to phytotrons for a short-term treatment with 350 and 700 μ l -1 CO2 for 4 wk in an atmosphere continuously labelled with 14CO2. No interactive effects on total net uptake of 14CO2 between long-term treatment and short-term treatment were observed. The short-term treatment with 700 μ 1-1 CO2 increased the total net uptake of 14CO2 by 22%, compared with the 350 μ l -1 CO2 treatment. The long-term pretreatment did not affect the total net uptake, suggesting that photosynthetic acclimation had not occurred. However, expressed per unit of needle mass a 14% reduction was observed in the trees pretreated at 700 μ l-1 CO2. This was not because of a reduced sink strength of the root system. This reduced uptake per unit of needle mass after long-term treatment may have implications for carbon storage in forest ecosystems. The results showed that an initial growth stimulation can eventually be annulled by developing physiological or morphological adaptions. 14CO2 in the root/soil respiration increased in the short-term treatment with 700 μ l-1 CO2, indicating a stimulated use of current carbon compounds either by roots or microorganisms. The water use efficiency during the short-term treatment with 700 μ l-1 CO2 increased by 32 %, but was not affected by the long-term pretreatment. Water use per unit needle mass during the short-term treatment was decreased both by the short-term treatment and by the long-term pretreatment by about 15%. Some of the observed effects appeared to be persistent, such as decreased water use per unit needle mass, whereas others, stimulation of total net 14CO2 uptake and water use efficiency, were transient.