Elevated CO2 increases photosynthetic rates, but many plants, including rice (Oryza sativa L.), lose photosynthetic capacity via decreases in amount and activity of ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO). Our objective was to determine the effect of both elevated CO2 and temperature on rice RuBisCO activity and carbamylation state. Rice (cv. IR-30) was grown at 330 ("ambient") and 660 (elevated) μmol CO2 mol-1 air, and at 40/33/37, 34/ 27/31, and 28/21/25°C daytime/nighttime/paddy temperatures, respectively. At 23 d after planting (DAP), leaves were sampled in predawn darkness and in midday sunlight. At 59 DAP, leaves were sampled eight times from predawn to sunset. In high sunlight at 23 DAP, carbamylation state, manifested by initial (in vivo) RuBisCO activity, remained constant at all temperatures in ambient CO2, but decreased with increasing daytime temperature from 28 to 40°C in elevated CO2. At 59 DAP, carbamylation state response to elevated CO2 and temperature at midday was confounded by variable sunlight. The total (Mg2+/CO3--activated) RuBisCO activity at midday was generally not affected by elevated CO2 at either DAP. Thus the endogenous inhibitor of RuBisCO, 2-carboxyarabinitol-1-phosphate (CA1P), was not responsible for changes of initial RuBisCO activity in response to elevated CO2 in high sunlight. In darkness at 23 DAP, under elevated CO2, percentage of potential maximum activity of RuBisCO was similar across all temperatures. Under ambient CO2, percentage of potential maximum activity of RuBisCO was similar to elevated CO2 at 34°C, but increased at 28°C and decreased at 40°C, due to changes in inhibition of RuBisCO by CA1P. RuBisCO protein per unit total soluble protein was generally not affected by CO2. Photosynthetic rates were always greater in elevated CO2. Thus, high CO2 overcame lack of complete carbamylation of RuBisCO in maintaining high rates of CO2 uptake at elevated temperatures.