A study was conducted in 21-year-old loblolly pine (Pinus taeda L.) trees growing in plantation in north central Georgia, USA. The experiment used branch chambers to impose treatments of ambient, ambient +165 and ambient + 330 μmol mol−1 CO2. After one growing season there was no indication of acclimation to elevated CO2. In August and September, carbon assimilation, measured by two different methods, was twice as high at ambient +330 μmol mol−1 CO2 than at ambient. Dark respiration was suppressed by 6% at ambient +165 and by 14% at ambient + 330 μmol mol−1 CO2. This suppression was immediate, and not an effect of exposure to elevated CO2 during growth, since respiration was reduced by the same amount in all treatments when measured at a high CO2 concentration. Elevated CO2 increased the growth of foliage and woody tissue. It also increased instantaneous transpiration efficiency, but it had no effect on stomatal conductance. Since the soil at the study site had low to moderate fertility, these results suggest that the growth potential of forests on many sites may be enhanced by global increases in atmospheric CO2, concentration.