The morphological and physiological response of the slender oat (Avena barbata Pott ex Link # AVEBA) to the herbicides barban (4-chloro-2-butynyl 3-chlorophenylcarbamate) and difenzoquat (1,2-dimethyl-3,5-diphenyl-1H-pyrazolium) in conjunction with decreased water availability was determined for seven populations, representing three ecotypes, under greenhouse conditions. Generally, within the range of sublethal herbicide doses, with increasing herbicide rates, phytotoxicity ratings increased, but plant dry weight, tiller height, and number of spikelets were decreased. Also, the number of juvenile tillers decreased, while that of fertile tillers increased. Flag leaf area increased and flowering was delayed. The ratio of number of spikelets to plant dry weight and seed weight was influenced the least. Under the highest rates of difenzoquat, the within-plant variance of spikelet number decreased, indicating that there may have been a more equal partitioning of resources amongst tillers for spikelet production. The general influence of water stress was to amplify the effect of the herbicide. For example, the dry treatment reduced dry weight and tiller height, and delayed flowering. Within a particular herbicide treatment, the effect of the water stress was to cause reduced within-plant variance for days to flowering, flag leaf area, and number of spikelets. Three reactions were observed that could have helped buffer decreases in spikelet production: 1) An increased fraction of the dry weight of the plants was partitioned into the spikelets at the expense of other vegetative matter, 2) the increased leaf area of the primary tiller may have helped counterbalance any reduction in photosynthesis caused by herbicide action, and 3) an increased number of juvenile tillers was converted into fertile tillers resulting in an increased number of mature tillers. These data indicate that the slender oat has a remarkable “phenotypic plasticity,” which enables it to maintain reproductive structures under sublethal herbicide doses.