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It is important to know how the increasing atmospheric concentration of carbon dioxide (CO2) will affect growth of agricultural plants. The objective of this study was to determine the effect of elevated CO2 on canopy photosynthetic rate of prairie (rangeland) plants growing under natural field conditions. The dominant plants were warm-season grasses with the C4 type of photosynthesis. Sixteen closed-top, cylindrical, plastic chambers (1.5 m in diameter; 1.8 m tall) were placed on the prairie to maintain two levels of CO2 (ambient and twice ambient) over a full growing season in 1990. The soil (silty clay loam) was kept at a high water (field capacity) or a low water level (no water added). Carbon dioxide concentration, air temperature, net radiation, canopy photosynthetic rate, and canopy evapotranspiration rate were measured in the 16 chambers on 49 sunny days during the season. The target value for high-CO2 chambers was 720 cm3 CO2 m-3; the measured mean concentrations varied from 710.8 to 720.1 cm3 CO2 m-3. For chambers with ambient CO2, the chamber-to-chamber variation was minor, with mean values ranging from 350.8 to 356.0 cm3 CO2 m-3. Daytime air temperatures at 100 cm aboveground in the chambered plots averaged 2.7°C warmer than outside. Early in the season, net radiation was usually similar among chambers with the different CO2 and water treatments, but late in the season, differences occurred among chambers, possibly because of the amount of tall grasses that shaded the radiometers. Under the high-water treatment, canopy photosynthesis of plants grown with doubled and ambient CO2 averaged 41.8 [mu]mol m-2 s-1 and 44.5 [mu]mol m-2 s-1, respectively. These results are consistent with previous findings, which showed that the photosynthetic rate of C4 plants on rangeland was not augmented when the CO2 concentration was increased. Under the low-water treatment, photosynthesis of plants grown with doubled CO2 was slightly more (36.9 [mu]mol m-2 s-1) than that of plants grown with ambient CO2 (31.7 [mu]mol m-2 s-1). This observation is in agreement with other results, which have shown that high CO2 alleviates water-stress effects on plants. Elevated CO2 reduced canopy evapotranspiration rate by 18 and 8%, under the high- and low-water levels, respectively. The results suggested that, as the CO2 concentration in the atmosphere increases, water lost from rangelands will be reduced.