The first step in photosynthetic carbon fixation, ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco, EC 4.1.1.39), catalyzes the uptake of CO₂ into sugars. The potential carbon fixation is reduced in the presence of oxygen since oxygen can successfully compete for the active site. This decreased efficiency of rubisco has been suggested to arise from the evolution of rubisco in a low-oxygen, carbon dioxide-rich atmosphere. Other photosynthetic schemes, such as C⁴ and CAM, have evolved that spatially or temporally remove the site of rubisco activity from the presence of oxygen, greatly increasing the carboxylation efficiency of the reaction. Under short term exposure to elevated levels of CO₂, C₃ plants grown at ambient CO₂ levels show increased net carbon uptake, yet the activity of rubisco is reduced. However, although plants grown in elevated CO₂ conditions for long periods have increased productivity, the anticipated increase is not realized, suggesting an adaptation of the photosynthetic machinery to the elevated CO₂ environment. In order to explore the response of photosynthetic activity in cotton to elevated CO₂ growth conditions, plants were grown in high CO₂ environments from germination. Plants were grown in naturally-lit temperature controlled growth cabinets in sand at 30°C day/22°C night. Plants were watered daily with half-strength Hoagland's nutrient solution. Carbon dioxide levels were maintained at 700 µl(.)l^-1 and 350 µl(.)l^-1 throughout the growth period of the plants. Prior to squaring, all treatments received half-strength Hoagland's nutrient solution. From squaring to flowering, plants were irrigated with 5 different nutrient solutions, 0, 1, 2, 6, and 10 mM N at each of the CO₂ treatments. All plants were watered three times a day. Canopy photosynthesis was determined throughout the growth period. Seven weeks after planting, the youngest fully expanded leaves from a minimum of three plants per treatment were selected for determination of individual leaf photosynthetic parameters. Leaf photosynthetic rates as a function of CO₂ and photon flux density were measured using a LiCor 6200 gas exchange system. After photosynthetic measurements, fully illuminated leaves were sampled using a metal punch immersed in liquid N that rapidly froze the leaves, stopping biochemical reactions and maintaining the enzymatic activation states in the in vivo state. Activities of photosynthetic regulatory enzymes were determined using spectrophotometric procedures. The individual leaf photosynthetic response to CO₂ showed no difference as a function of growth conditions, indicating no difference in the initial carboxylation reaction. Maximal leaf photosynthetic capacity, however, was linearly dependent on leaf nitrogen per unit area in both ambient and twice-ambient CO₂ grown plants. Plants grown at elevated CO₂ assimilated about 34% more carbon than plants grown at present-day CO₂ levels.