In previous investigations two anion conductances were discovered in guard-cell protoplasts: the quickly activating anion conductance (QUAC, R-type) and the slowly activating anion conductance (SLAC, S-type). In this investigation, effects of malate on the two anion conductances were tested in whole guard cells of Vicia faba L. by the use of the discontinuous single-electrode voltage-clamp method. Application of 1-s voltage ramps proved that QUAC displayed the malate shift of the activation threshold toward hyperpolarization also in complete guard cells. The sensitivity of SLAC to external malate was determined by responses to voltage pulses of 20 s duration at Cl- concentrations of 0.1, 3 or 50 mM. At no voltage were the currents measured at the end of the pulses in the presence and absence of malate significantly different from each other; the current-voltage relationship of SLAC appeared not to be affected by malate. However, in 32% of the cells exposed to malate, current activation in response to voltage steps occurred within 0.1 s, faster than was typical for SLAC, and activation was followed by inactivation with a half-time similar to 10 s: SLAC apparently had changed to QUAC. Simultaneously, the free-running membrane voltage depolarized at 0.1 mM Cl-, did not change at 3 mM Cl- and polarized at 50 mM Cl-, indicating that activation of QUAC increased the membrane conductance for anions and thereby drove the membrane voltage toward the equilibrium voltage of Cl-. The malate-induced changes were fully reversible at Cl- concentrations of 0.1 and 3 mM. These results reinforce the proposition that SLAC and QUAC represented two switching modes of the same anion channel (however, they do not suffice as proof); they also show that this interconvertibility can enable guard cells to control their membrane voltage rapidly.