Determining effects of drought on prairie grasses could help us understand shifts in plant distributions and abundances with altered precipitation. The mixed grass prairie of North America is the zone where tallgrass prairies of the east intermingle with shortgrass prairies of the west. Species transitions in the mixed grass prairie reflect precipitation gradients, but little is known regarding how dominant grasses respond physiologically to water availability. We measured the performance of Andropogon gerardii, Bouteloua curtipendula, Schizachyrium scoparium, and Sorghastrum nutans in three precipitation treatments (ambient rainfall, drought, irrigated) in a southern mixed grass prairie near Hays, Kansas, USA. Photosynthesis and water potential were measured monthly in June and July of 2010 and 2011. Phenology and height measurements were collected semimonthly in June, July, and August of 2011. Growth increased under irrigated treatments in all four species. Conversely, leaf δ¹³C became lower in drought treatments. The highest photosynthesis rates were measured in irrigated S. nutans, but S. nutans was the most impacted by drought. No individuals of S. nutans remained in drought treatments by August 2011, leaf δ¹³C and water potentials were significantly higher than in other species, and individuals of S. nutans only reached reproductive stage in irrigated treatments. The most drought tolerant species were B. curtipendula and S. scoparium, as indicated by lower leaf δ¹³C and water potentials and an ability to maintain higher stomatal conductance and photosynthesis with low water availability. Moreover, individuals of B. curtipendula and S. scoparium reached reproductive stage in ambient and irrigated treatments. Andropogon gerardii had intermediate drought tolerance, as indicated by intermediate photosynthesis, leaf δ¹³C, and water potential, and individuals only reaching reproductive stage in irrigated treatments. Physiological differences among dominant prairie grasses under decreased water availability could help us understand mechanisms of drought tolerance and how climate change might impact native grasslands.