Net photosynthetic rates (Pns) in leaves were compared between rice plants grown in ambient air control and free-air CO2 enrichment (FACE, about 200 micromol mol(-1) above ambient) treatment rings. When measured at the same CO2 concentration, the Pn of FACE leaves decreased significantly, indicating that photosynthetic acclimation to high CO2 occurs. Although stomatal conductance (Gs) in FACE leaves was markedly decreased, intercellular CO2 concentrations (Ci) were almost the same in FACE and ambient leaves, indicating that the photosynthetic acclimation is not caused by the decreased Gs. Furthermore, carboxylation efficiency and maximal Pn, both light and CO2-saturated Pn, were decreased in FACE leaves, as shown by the Pn-Ci curves. In addition, the soluble protein, Rubisco (ribulose-1,5-bisphosphate caboxylase/oxygenase), and its activase contents as well as the sucrose-phosphate synthase activity decreased significantly, while some soluble sugar, inorganic phosphate, chlorophyll and light-harvesting complex II (LHC II) contents increased in FACE leaves. It appears that the photosynthetic acclimation in rice leaves is related to both ribulose-1,5-bisphosphate (RuBP) carboxylation limitation and RuBP regeneration limitation.