Metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] at 1 × 10−5 and 1 × 10−4 M increased the leakage of previously absorbed, 32P-labeled orthophosphate from the roots of onion (Allium cepa L.), a susceptible species, by 14 and 41 times the control values, respectively. A significant amount of 32P leaked from the roots of the moderately susceptible species, cotton (Gossypium hirsutum L. ‘DPL 61′) and cucumber (Cucumis sativus L. ‘Ashley′), whereas no significant loss of 32P occurred from two tolerant species, soybean [Glycine max (L.) Merr. ‘Bragg′] and corn (Zea mays L. ‘Pioneer 3369A′). At either 1 × 10−7 or 1 × 10−6 M, 1,8-naphthalic anhydride (NA) prevented 32P leakage from onion roots in the presence of 1 × 10−5 M metolachlor. High concentrations of NA [0.1% (w/v) suspensions], however, promoted 32P leakage and did not protect onion roots from the leakage induced by high concentrations (1 × 10−4 M) of metolachlor. Neither metolachlor nor alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide], at 1 × 10−4 M, inhibited the uptake of acetate-2-14C or malonic acid-2-14C into excised cotton root tips or the incorporation of the precursors into lipids. Similarly, neither herbicide inhibited phospholipid synthesis by cotton root tips. Incorporation of 14C-choline chloride into phosphatidylcholine was not significantly inhibited by metolachlor.