We examined the response of stomatal conductance (gs) to increasing leaf to air vapor pressure difference (D) in early season C3 (Bromus japonicus) and late season C4 (Bothriochloa ischaemum) grasses grown in the field across a range of [CO2] (200 - 550 �mol mol-1). Stomatal sensitivity to D was calculated the slope of the response of gs to the natural log of externally manipulated D (dgs/dlnD). Increasing D and [CO2] significantly reduced gs in both species. Increasing [CO2] caused a significant decrease in stomatal sensitivity to D in Br. japonicus, but not in Bo. ischaemum. The decrease in stomatal sensitivity to D at high [CO2] for Br. japonicus fit theoretical expectations of a hydraulic model of stomatal regulation, in which gs varies to maintain constant leaf water potential. The weaker stomatal sensitivity to D in Bo. ischaemum suggested that stomatal regulation of leaf water potential was poor in this species, or that non-hydraulic signals influenced guard cell behavior. The decline in gs with increasing D reduced net photosynthesis (A) in both species, but increased stomatal limitation of photosynthesis only in Br. japonicus. Rising [CO2] had the greatest effect on gs and A in Br. japonicus at low D. In contast, the strength of stomatal and photosynthetic responses to CO2 were not affected by D in Bo. ischaemum. These species dominate the seasonal transition from early C3 to late C4 production in this grassland. Interspecific variation in the response of stomatal conductance to D therefore has implications for predicting seasonal ecosystem responses to CO2.