The translocation and root exudation of leaf-applied sublethal concentrations of 14C-labeled clopyralid (3,6-dichloropicolinic acid) and 14C-labeled 2,4-D [(2,4-dichlorophenoxy)acetic acid] were compared in Canada thistle [Cirsium arvense (L.) Scop. var. horridum Wimm. and Grab. ♯ CIRAR] at the rosette stage over a period of 9 days. The rate of absorption and export of 14C out of the treated leaf was similar for both herbicides. However, the distribution of the herbicides throughout the plant was very different. After 9 days, 15 vs. 3% of the applied 14C from 14C-clopyralid vs. 14C-2,4-D, respectively, was isolated from the foliage of the treated leaf. In the roots, twice as much 14C was recovered from the 14C-clopyralid treatments as from the 14C-2,4-D treatment at all sampling times, with 33 vs. 15% being recovered, respectively, after 9 days. When the plants were grown hydroponically, 20% of the 14C-clopyralid vs. 48% of the 14C-2,4-D was recovered in the nutrient solution during a 9-day period. No metabolites of either herbicide were recovered from the foliage, root system, or nutrient solution. Large differences in translocation of these herbicides may account for the unequal toxicity to young Canada thistle plants.