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To model the effect of increasing atmospheric CO2 on semi-arid grasslands, the gas exchange responses of leaves to seasonal changes in soil water, and how they are modified by CO2, must be understood for C-3 and C-4 species that grow in the same area. In this study, open-top chambers were used to investigate the photosynthetic and stomatal responses of Pascopyrum smithii (C-3) and Bouteloua gracilis (C-4) grown at 360 (ambient CO2) and 720 mumol mol(-1) CO2 (elevated CO2) in a semi-arid shortgrass steppe. Assimilation rate (A) and stomatal conductance (g(s)) at the treatment CO2 concentrations and at a range of intercellular CO2 concentrations and leaf water potentials (psi(leaf)) were measured over 4 years with variable soil water content caused by season and CO2 treatment. Carboxylation efficiency of ribulose bisphosphate carboxylase/oxygenase, (V-c,V-max), and ribulose bisphosphate regeneration capacity (J(max)) were reduced in P. smithii grown in elevated CO2, to the degree that A was similar in elevated and ambient CO2 (when soil moisture was adequate). Photosynthetic capacity was not reduced in B. gracilis under elevated CO2, but A was nearly saturated at ambient CO2. There were no stomatal adaptations independent of photosynthetic acclimation. Although photosynthetic capacity was reduced in P. smithii growing in elevated CO2, reduced gs and transpiration improved soil water content and psi(leaf) in the elevated CO2 chambers, thereby improving A of both species during dry periods. These results suggest that photosynthetic responses Of C-3 and C-4 grasses in this semi-arid ecosystem will be driven primarily by the effect of elevated CO2 on plant and soil water relations.