Leaves treated with acifluorfen [sodium 5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoate] contain greatly increased levels of N-feruloyl-3-methoxytyramine (spinach) and of 10 phytoalexins, i.e., glyceollins I, II, and III and glyceofuran (soybean), phaseollin (bean and pinto bean), pisatin (pea), medicarpin and wyerone (broad bean), xanthotoxin (celery), and hemigossypol (cotton). Enhanced synthesis of these compounds is related to the acifluorfen concentration and exposure time to light. The phytotoxicity of acifluorfen and oxyfluorfen [2-chloro-l-(3-ethoxy-4-nitrophenoxy)-4-(trifluoromethyl)benzene] to spinach is counteracted by appropriate treatments with (aminooxy)acetic acid, L-2-(aminooxy)-3-phenylpropionic acid, or silver nitrate and by heat shock. Under certain conditions soybean injury is ameliorated by combining (aminooxy)acetic acid with acifluorfen and silver nitrate with oxyfluorfen. These relationships for diphenyl ether (DPE) herbicides and protective treatments resemble those for other stress factors with associated increases in lipid peroxidation, membrane permeability, ethylene production, and phenylalanine ammonia-lyase (PAL) activity. Although increased PAL activity is not a primary lesion, it may play an important role in DPE herbicide action.