Periodic long-lasting floods are a major component of the natural disturbance regime affecting grassland vegetation in the Flooding Pampa, Argentina. Chronic grazing by livestock imposes a different, and potentially conflicting, environmental constraint on vegetation structure in this system. Grazing reduces graminoid biomass and increases the abundance of forb species. We test the hypothesis that, in the short term, prolonged flooding would revert the effects of grazing on plant community structure. Flooding impacts on community biomass, canopy architecture, and species composition of formerly grazed grassland were examined over a 6-wk period using soil monoliths extracted from the field with intact vegetation (mesocosms). Forb and graminoid species co-dominated the unflooded, control mesocosms, while forb biomass declined strongly with prolonged flooding. Graminoids overcompensated for the decrease in forb biomass under flooding conditions. Total above-ground biomass was 75% greater in flooded mesocosms; on average, graminoids accounted for 93% of the plant biomass in these communities. Flooding changed the initially flat foliage structure for a taller and more complex canopy, with graminoid species overtopping the remaining forbs. Total below-ground phytomass was not affected by flooding; however, no live roots of the dominant forb in unflooded vegetation, Ambrosia tenuifolia, were found in flooded mesocosms. Thus, flood-resistant graminoids occupied most of the canopy and soil space vacated by least tolerant forbs. Community responses to flooding would have reflected two possible mechanisms: (1) direct deleterious effects on non-tolerant forbs due to soil oxygen deprivation; and (2) indirect effects involving either intensification (forbs) or amelioration (grasses) of competitive interactions. Overall, this mesocosm experiment supports previous field observations indicating that severe floods may rapidly revert vegetation traits typically promoted by domestic grazing. Periodic flooding could be regarded as a natural subsidy rather than a stressor for the maintenance of this grassland ecosystem.