The relationship between carbon exchange rate (CER) and internal CO2 concentration was measured in leaves of saplings of Liquidambar styraciflua L. (sweetgum) and Pinus taeda L. (loblolly pine) grown from seed for more than 14 months at atmospheric CO2 concentrations of either 350 or 500 μl l−1. An elevated concentration of CO2 during growth reduced CER at any given internal CO2 concentration in sweetgum, but not in loblolly pine. Stomatal limitation of CER showed little response to concentration of CO2 during measurement, but was higher in both species when grown at 500 than at 350 μl l−1 CO2. The net effect of a long-term increase in CO2 concentration from 350 to 500 μl l−1 was an increase in CER of loblolly pine, but a slight decrease in CER of sweetgum. It is suggested that the depression of CER in sweetgum resulted from a reduction in the activity of ribulose-1,5-bisphosphate carboxylase-oxygenase.