Growth at an elevated CO(2) concentration resulted in an enhanced capacity for soybean (Glycine max L. Merr. cv Bragg) leaflet photosynthesis. Plants were grown from seed in outdoor controlled-environment chambers under natural solar irradiance. Photosynthetic rates, measured during the seed filling stage, were up to 150% greater with leaflets grown at 660 compared to 330 microliters of CO(2) per liter when measured across a range of intercellular CO(2) concentrations and irradiance. Soybean plants grown at elevated CO(2) concentrations had heavier pod weights per plant, 44% heavier with 660 compared to 330 microliters of CO(2) per liter grown plants, and also greater specific leaf weights. Ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco) activity showed no response (mean activity of 96 micromoles of CO(2) per square meter per second expressed on a leaflet area basis) to short-term ( approximately 1 hour) exposures to a range of CO(2) concentrations (110-880 microliters per liter), nor was a response of activity (mean activity of 1.01 micromoles of CO(2) per minute per milligram of protein) to growth CO(2) concentration (160-990 microliters per liter) observed. The amount of rubisco protein was constant, as growth CO(2) concentration was varied, and averaged 55% of the total leaflet soluble protein. Although CO(2) is required for activation of rubisco, results indicated that within the range of CO(2) concentrations used (110-990 microliters per liter), rubisco activity in soybean leaflets, in the light, was not regulated by CO(2).