Branches of 22-year-old loblolly pine (Pinus taeda, L.) trees growing in a plantation were exposed to ambient CO2, ambient + 165 μmol mol−1 CO2 or ambient + 330 μmol mol−1 CO2 concentrations in combination with ambient or ambient + 2°C air temperatures for 3 years. Field measurements in the third year indicated that net carbon assimilation was enhanced in the elevated CO2 treatments in all seasons. On the basis of A/Ci, curves, there was no indication of photosynthetic down-regulation. Branch growth and leaf area also increased significantly in the elevated CO2 treatments. The imposed 2°C increase in air temperature only had slight effects on net assimilation and growth. Compared with the ambient CO2 treatment, rates of net assimilation were ∼1·6 times greater in the ambient + 165 μmol mol−1 CO2 treatment and 2·2 times greater in the ambient + 330 μmol mol−1 CO2 treatment. These ratios did not change appreciably in measurements made in all four seasons even though mean ambient air temperatures during the measurement periods ranged from 12·6 to 28·2°C. This indicated that the effect of elevated CO2 concentrations on net assimilation under field conditions was primarily additive. The results also indicated that the effect of elevated CO2 (+ 165 or + 330 μmol mol−1) was much greater than the effect of a 2°C increase in air temperature on net assimilation and growth in this species.