Substituted quinolinecarboxylic acids, a new class of highly selective auxin herbicides, are reviewed. Quinclorac is used in rice to control dicotyledonous and monocotyledonous weeds, particularly barnyardgrass (Echinochloa crus-galli). Quinclorac has also been developed for use in turfgrass areas, spring wheat and chemical fallow. Quinclorac is readily absorbed by germinating seeds, roots, and leaves and is translocated acropetally and basipetally. Quinclorac mimics an auxin overdose, stimulating the induction of 1- aminocyclopropane-1-carboxylic acid (ACC) synthase activity, which promotes ethylene biosynthesis. In susceptible dicotyledons, increased ethylene levels trigger abscisic acid (ABA) accumulation, which plays a major role in growth inhibition and the induction of epinasty and senescence. In sensitive grasses, tissue cyanide accumulates, formed as a co-product during increased ACC and ethylene synthesis. Cyanide causes phytotoxicity, characterized by root and shoot growth inhibition with tissue chlorosis and necrosis. These effects were not observed in tolerant rice and a resistant biotype of barnyardgrass. No significant differences in uptake, translocation, or metabolism of quinclorac between resistant and sensitive grasses were found. Hence, a target-site-based mechanism of selectivity is suggested. The induction of ACC synthase activity is concluded to play the primary role in the selective herbicide action.