Inhibition of root elongation and modification of membrane properties are sensitive responses of plants to aluminium. The present paper reports on the effect of Al on lipid peroxidation and activities of enzymes related to production of activated oxygen species. Soybean seedlings (Glycine max L. cv. Sito) were precultured in solution culture for 3-5 days and then treated for 1-72 h with Al (AlCl3) concentrations ranging from 10 to 75-mu-M at a constant pH of 4.1. In response to Al supply, lipid peroxidation in the root tips (< 2 cm) was enhanced only after longer durations of treatment. Aluminium-dependent increase in lipid peroxidation was intensified by Fe2+ (FeSO4). A close relationship existed between lipid peroxidation and inhibition of root-elongation rate induced by Al and/or Fe toxicity and/or Ca deficiency. Besides enhancement of lipid peroxidation in the crude extracts of root tips due to Al, the activities of superoxide dismutase (EC 188.8.131.52) and peroxidase (EC 184.108.40.206) increased, whereas catalase (EC 220.127.116.11) activity decreased. This indicates a greater generation of oxygen free radicals and related tissue damage. The results suggest that lipid peroxidation is part of the overall expression of Al toxicity in roots and that enhanced lipid peroxidation by oxygen free radicals is a consequence of primary effects of Al on membrane structure.