The physiological reasons for the inhibition of root elongation by Al are not yet;understood. The objective of the present studies was to clarify short-term effects of Al on plasma-membrane integrity of soybean (Glycine max L.) roots using different physiological/biochemical parameters. Initial response (0 - 3 h) of roots to Al (50-mu-M AlCl3) was a reduction of K+ net-efflux. This was most pronounced at low Ca supply (25 versus 2500-mu-M). Using Rb-86, it could be shown that Al effects on K+ net-efflux were mainly due to reduced K+ efflux whereas effects on K+ influx were negligible. After 2 h of Al treatment, induction by Al of non-constitutive callose synthesis could be demonstrated. Lipid peroxidation was enhanced after 32 h of Al treatment especially in the presence of higher Fe2+ (FeSO4) concentrations. Fe(III) reducing capacity of Fe-deficient plants, as an indicator of high activity of plasma membrane-bound processes, was not impaired but rather enhanced by short-term Al treatment. The results clearly indicate, that Al concentrations leading to severe inhibition of root elongation do not primarily lead to membrane disruption. However, specific membrane functions are modified by Al.