Shattercane (Sorghum bicolor (L.) Moench) and woolly cupgrass (Eriochloa villosa (Thunb.) Kunth) growing under water stress conditions suffer morphological and physiological changes that may affect the activity of postemergence herbicides. Morphological and physiological changes in shattercane and woolly cupgrass growing under different levels of water stress were determined. Leaf area expansion was the growth variable most affected in both species. Both plants exhibit decreased dry matter accumulation due to water stress treatments. Woolly cupgrass stomatal conductance was more affected by water stress than shattercane. Photosynthesis was greatly reduced in both weeds, but only partially related to decreased stomatal conductance, indicating the possibility than non-stomatal factors superimpose their action on stomatal control, thus reducing the carbon exchange rate. Maximum accumulation of epicuticular wax was determined for shattercane and woolly cupgrass plants growing under moderate water stress conditions (-0.4 MPa of soil water potential). Abscisic acid content of shattercane and woolly cupgrass leaves and indole-3-acetic acid content of woolly cupgrass leaves increased with increasing intensity and duration of water stress. Additionally, the effect of differential water status on the activity, retention, uptake, and translocation of nicosulfuron (2-(((4,6-dimethoxypyrimidin-2-yl) aminocarbonyl) aminosulfonyl) -N,N-dimethyl-3-pyridinecarboxamide), and primisulfuron-methyl (3-(4,6-bis-(difluoromethoxy) -pyrimidin-2-yl) -1-(2-methoxycarbonylphenylsulfonyl)urea) was studied. Maximum herbicide activity was demonstrated on well-watered plants. Increasing plant water stress reduced herbicide activity in both species. Shattercane uptake of nicosulfuron and primisulfuron-methyl was not affected by soil water potential. However, woolly cupgrass uptake of both herbicides was affected by soil water potential, demonstrating reduced herbicide absorption due to water stress. Both species accumulated herbicides in the cuticles. The total amount of herbicide translocated was significantly affected by water stress treatments in both species.