Perchlorate uptake has been reported in a variety of vascular plants, but the mechanisms affecting the uptake, translocation, and exudation of ClO 4- in plants are not well known. The uptake and transport behavior of ClO4- was studied in smartweed (Polygonum spp.) grown in aerated hydroponic ClO4- solutions with or without other common anions. Uptake followed first-order kinetics (0.017 ± 0.003 day-1) and there was no apparent competitive effect on ClO4- uptake for up to 500 mg L-1 of NO 3--N, SO42-, PO4 3-, or Cl-. The net ClO4- mass uptake was proportional to transpiration but with some exclusion of ClO 4- from the transpiration stream. Up to 4% of the ClO 4- in plants after a 30-d exposure to a 10-mg/L solution of ClO4- was exuded from the roots within 15 d after the exposure was ended. Perchlorate exudation followed first-order kinetics with a rate coefficient of 0.0018 ± 0.0006 (day-1). Results obtained in this study indicate that evapotranspiration may be the critical variable to predict accumulation potential in plants and that phytoremediation strategies should take into account the re-release potential of ClO4- via exudation.