Abstract. The effects were studied of season-long (75 and 88d) exposure of rice (Oryza sativa L. cv. IR-30) to a range of atmospheric CO2 concentrations in outdoor, computer-controlled, environment chambers under natural solar radiation. The CO2 concentrations were maintained at 160, 250, 330, 500, 660 and 900μmol mol-1 air. Photosynthesis increased with increasing growth CO2 concentrations up to 500u.mol moP1, but levelled off at higher CO2 values. Specific leaf area also increased significantly with increasing CO2. Although leaf dry weight and leaf area index increased, the overall response was not statistically significant. Leaf nitrogen content dropped slightly with elevated CO2, but the response was not statistically significant. The specific activity of ribulose bisphosphate carboxylase/oxygenase (rubisco) declined significantly over the CO2 concentration range 160 to 900μmol mol-1. When expressed on a leaf area basis, rubisco activity decreased by 66%. This was accompanied by a 32% decrease in the amount of rubisco protein as a fraction of the total soluble leaf protein, and by 60% on a leaf area basis. For leaves in the dark, the total rubisco activity (CO2/Mg2+-activated) was reduced by more than 60%. This indicates that rice accumulated an inhibitor in the dark, probably 2-car-boxyarabinitol 1-phosphate (CA-1-P). However, the inhibitor did not seem to be involved in the acclimation response. The degree of carbamylation of the rubisco enzyme was unchanged by the CO2 growth regime, except at 900 [μmol mol-1 where it was reduced by 24%. The acclimation of rice to different atmospheric CO2 conditions involved the modulation of both the activity and amount of rubisco protein in the leaf.