Variations in water relations and stomatal response of Quercus pubescens Willd. were analyzed under Mediterranean field conditions during two consecutive summers (1993 and 1994) at two locations characterized by different atmospheric CO(2) concentrations because of the presence at one of them of a CO(2) spring. Trees at the CO(2) spring site have been growing for generations in elevated atmospheric CO(2) concentrations. The heat-pulse velocity technique was used to estimate water use of trees during a 5-month period from June to November 1994. At the end of the sap flow measurements, the trees were harvested and foliage and sapwood area measured. At both sites, maximum leaf conductance was related to predawn shoot water potential. Effects of summer drought on plant water relations, including whole-plant transpiration, were severe, but leaf conductance and water potential recovered to predrought values after major rainfall in September. Leaf conductance, predawn water potential, and sometimes sap flow, decreased in parallel with increases in hydraulic resistance, reaching a minimum in midsummer. Hydraulic resistance was higher in trees at the control site than in trees at the CO(2) spring site. The effect of elevated CO(2) concentration on leaf conductance was less at high leaf-to-air water vapor pressure difference than at low leaf-to-air water vapor pressure difference. Mean and diurnal sap fluxes were consistently higher in trees at the control site than in trees at the CO(2) spring site. During the summer period, plant water use varied between the two sites. Trees at the control site had consistently higher sap flow at corresponding values of sapwood cross-sectional area than trees at the CO(2) spring site. Because trees at the CO(2) spring site supported a smaller foliage area for a corresponding sapwood cross-sectional area than trees at the control site, the overall mean sap flux/foliage area ratio did not differ between sites. The results are discussed in terms of effects of elevated CO(2) concentration on plant water use at the organ and whole-tree scale.