The activity of lipoxygenase (EC 1.13.11.12) in Arabidopsis thaliana (L.) Heynh seedlings and mature plants was estimated spectrophotometrically at 234 nm. Linoleic acid was used as a substrate. Lipoxygenase activity showed two pH optima: at 7.0 and 10.0 in seedlings, and at pH 8.0 and 10.0 in leaves of mature plants. Seven-week-old plants were transferred to a hydroponic system and treated with different concentrations of Cd(2+) or Cu(2+) [in microM]: 0, 5, 25, 50, 100 for 7 days. The lipoxygenase activities at pH 8.0 and 10.0 depended on the metal that was added to the nutrient solution. The main change in lipoxygenase activity was under Cd(2+)stress at pH 8.0 and under Cu(2+)excess at pH 10.0.