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Effects of diphenyl ether (DPE) and DPE-type herbicides on porphyrin biosynthesis in cucumber seedlings were studied by ion-pair HPLC. The results: (a) Acifluorfen-methyl (AFM) did not increase the protoporphyrin IX (Proto IX) content in either etiolated cotyledons or white hypocotyls in the presence of 2, 2-dipyridyl, which was synergetic with AFM in stimulating the accumulation of Proto IX in green cotyledons, although AFM has been reported to cause the accumulation of Proto IX by inhibiting protoporphyrinogen oxidase or stimulating ALA-synthesizing enzymes. AFM hardly affected the protochlorophyllide (Pchlide) level either, contrary to the hypothesis that AFM stimulates ALA-synthesizing enzymes. (b) HPLC chromatograms of porphyrin pigments indicated that nitrof en acts in the same way as AFM. (c) The light-independent m-substituted DPEs tested, the mode of action of which has not yet been clarified, stimulated the accumulation of Mg-Proto IX monomethyl ester both in green cotyledons and in white hypocotyls when combined with 2, 2-dipyridyl. (d) Two DPE-type compounds, 3-chloro-2-[4-chloro-2-fluoro-5-(2-propynyloxy) phenyl]-4, 5, 6, 7-tetrahydro-2Hindazole (S-275) and ethyl 2-[4-nitro-l-(2, 3, 4-trichlorophenyl)pyrazoryl-5-oxy]propionate (OC-17595), caused Proto IX to accumulate in green cotyledons. This result strongly suggested that these compounds, although different from AFM in chemical structure, have the same mode of action as AFM. (e) 1, 3-Dimethyl-4-(2, 4-dichlorobenzoyl)-5-hydroxypyrazole (DTP), a Pchlide inhibitor, neither decreased Pchlide nor increased Proto IX, which disagreed with the previous result.