A potential-sensitive fluorescent probe was used to study the effect of IAA, 1-NAA, and their inactive analogues (indole and 2-NAA) on the transport of K+ and Ca2+ across the plasmalemma of plant cells. The ion transport was evident from membrane potential changes induced by the imposed transmembrane gradients for K+ or Ca2+. When the plasmalemma vesicles were loaded with 150 mM K2SO4 and placed into a medium containing 150 mM Na2SO4, the addition of 10-6 M IAA or 1-NAA to the incubation medium induced changes in the membrane potential due to the efflux of K+ from the vesicles. In the absence of K+ gradient (equimolar concentrations of K+ inside and outside the vesicles), the effect of IAA and 1-NAA on dye fluorescence was comparable to the small unspecific effect of indole and 2-NAA. To study the auxin-dependent Ca2+ transport, vesicles were loaded with 150 mM K2SO4 and 1 mM CaSO4 and placed into the Na+-containing medium. In these experiments, the membrane potential was changed by addition of IAA and 1-NAA,