Addition of aluminium chloride (50 μM Al) caused different effects on the transmembrane electrical potential (PD) of root cells in Al-tolerant wheat (Triticum aestivum) cv. Kadett and Al-sensitive cv. WW 20299. As changes in PD of plant cells may depend on transient fluxes of protons, potassium and/or calcium through cell membranes, the effect of Al was investigated on the cytosolic concentrations of these ions in protoplasts isolated from root tips of the same cultivars. The tetra[acetoxymethyl] esters of the fluorescent dyes bis-carboxyethyl-carboxyfluorescein, BCECF, K+-binding benzofuran isophthalate, PBFI, and the stilbene chromophore Fura 2-AM were used to determine pH, K+ and Ca2+, respectively. Changes in fluorescence ratios, directly reflecting changes in [H+], [K+] and [Ca2+] in the cytosol, were determined by photometry fluorescence microscopy. Additions and removals of Al to and from both cultivars caused hyperpolarizations and depolarizations, respectively, but only in the sensitive cv. WW 20299 did the resting PD decrease gradually. Addition of Al to the protoplasts caused rapid changes in cytosolic pH, free [K+] and [Ca2+]. In both cultivars Al caused a transient oscillating increase in cytosolic [Ca2+] for 1 or 2 min and a rapid pH-dependent change in cytosolic [K+]. At pH 5 the presence of K+ in the medium diminished the Al-induced decrease in cytosolic [K+]. Aluminium (50 μM) induced a transient increase in cytosolic [H+] (pH decreased) in both cultivars, but the cytosolic pH returned to its initial value only in the Al-tolerant cv. Kadett. In the Alsensitive cv. WW 20299, repeated additions of Al caused a gradual decline in pH. Moreover, in the presence of 1 mM KCl, pH recovered completely in both cultivars. Since only the effect on pH differed in the two cultivars, the more toxic effect of Al on the cv. WW 20299 should be related to the change in pH.