The effect of diclofop-methyl on lipid synthesis in entire oat ( Avena sativa L. "Prior") and corn ( Zea mays L. "3377 Pioneer") roots, in 1-cm corn root tips, and in corn shoots were examined at 24-hr intervals within a 4-day period. In addition, the de novo synthesis of specific lipids was compared in untreated and diclofop-methyl-treated whole roots and 1-cm root tips of corn seedlings. The overall rate of [ 14C]acetic acid incorporation into lipids in the entire root system of both oat and corn was not affected by diclofop-methyl. This response was evident up to 96 hr after treatment, despite a complete reduction in root growth 24 hr after herbicide exposure. In contrast, incorporation of [ 14C]acetic acid into lipids was greatly reduced in corn root tips (70% inhibition) after 24 hr and in corn shoots (52% inhibition) after a 48-hr treatment period. Thin-layer chromatographic separation and subsequent quantification of radiolabeled lipids indicated that 0.8 μ M diclofop-methyl primarily inhibited the synthesis of di- and triglycerides in both root tips and in intact roots. However, the incorporation of [ 14C]acetic acid into several other lipids, including glycolipids and phospholipids, was stimulated in mature root tissues treated with diclofop-methyl. This supports the view that mature root tissues of sensitive species are not damaged by growth-inhibiting concentrations of diclofop-methyl and that herbicidal activity must be coupled to growth processes.