The effects of ABA, isobutyric acid (IBA) and nicotine on K+ and Cl- efflux were studied in Arabidopsis thaliana seedlings, and the role of pHcyt and Em in the regulation of the efflux of these ions was discussed. The data show that treatments with IBA and nicotine influenced in opposite directions the efflux of either K+ or Cl-: K+ efflux was increased by nicotine and reduced in the presence of IBA, whereas Cl- efflux was stimulated by IBA and decreased by nicotine treatment. Under all the conditions tested ABA induced cytoplasmic acidification and inhibition of K+ and Cl- net efflux. Experiments aimed to estimate the individual contribution of pHcyt and Em in modulating K+ efflux indicated that, within the range of acidic pHcyt values, a regulation of K+ efflux was imposed by pHcyt on the control exerted by Em, the efflux being inhibited by lower pHcyt values. Conversely, in the alkaline side of pHcyt K+ efflux seemed linked only to the Em values. These results are consistent with the hypothesis that the decrease in K+ efflux observed in non-stomatal tissues in the presence of ABA may be mediated by the cytoplasmic acidification induced by the hormone.