The influence of the auxins indole-3-acetic acid (IAA) and 1-napthylene acetic acid (NAA) on K+ channels and their control was examined in stomatal guard cells of Vicia faba L. Intact guard cells were impaled with multibarrelled microelectrodes to record membrane potentials and to monitor K+ channel currents under voltage clamp during exposures to 0.1–100 µM IAA and NAA. Following impalements, challenge with either IAA or NAA in the presence of 10 mM KCl resulted in the concerted modulation of at least four different currents with distinct kinetic characteristics and concentration dependencies. Equivalent concentrations of benzoic acid were wholly without effect. Most striking, current carried by inward-rectifying K+ channels (IK,in) exhibited a bimodal response to both IAA and NAA which was reversed on washing the auxins from the bathing medium. The steady-state current was augmented 1.3- to 2-fold at concentrations between 0.1 and 10 µM and antagonized at concentrations near 30 µM and above. Auxin agonism of IK,in was time- and voltage-independent. By contrast, IK,in inactivation at the higher auxin concentrations was marked by a voltage-dependence and slowing of the kinetics for current activation. Inactivation of IK,in by the auxins was relieved when cytoplasmic pH (pHi) was clamped near 7.0 in the presence of 30 mM Na+-butyrate. In addition to the control of IK,in, current carried by a second class of (outward-rectifying) K+ channels rose in a monotonic and largely voltage-independent manner with auxin concentrations about 10 µM and above, and IAA and NAA also activated an inward-going current with a voltage dependence characteristic of guard cell anion channels. Further changes in background current were consistent with a limited activation of the H+-ATPase. Over the concentration range examined, the auxins evoked membrane hyperpolarizations and depolarizations of up to ±12–19 mV, depending on the free-running membrane potential prevailing before auxin additions. Prolonging exposures to 100 µM auxin beyond 3–5 min frequently elicited rapid transitions to voltages near EK as well as regenerative action potentials. However, in every case the voltage response was a predictable consequence of auxin action on the K+ channels and, at 100 µM auxin, on the anion current. These results demonstrate a control of K+ channel activity by auxin, consistent with the roles of these channels in mediating K+ flux for stomatal movements; the data associate a bimodal characteristic with the activity of IK,in, implicating pHi as a putative intermediate in its control, and offer strong evidence for a multiplicity of signal cascades evoked by auxin; finally, they highlight a coordinate modulation of transport activities by auxin, thereby drawing a close analogy to the pattern of stimulus-response coupling in abscisic acid.