Translocation of phloem-mobile herbicides was inhibited by their phytotoxic action on processes that maintain assimilate translocation. Glyphosate lowered import into developing sink leaves soon after it was applied to exporting sugarbeet leaves. Later, photosynthesis slowed down and starch accumulation stopped, but export of both assimilate and glyphosate continued until it was limited by starch availability at night. Experiments with field pennycress and Tartary buckwheat indicated that self-limitation of chlorsulfuron translocation probably occurred and that it resulted from lowered assimilate entry into phloem rather than from inhibition of photosynthesis or carbon allocation. Leakage of chlorsulfuron from the phloem when export was slowed down also may have contributed to its reduced translocation.