The efficacy and mechanism of alpha-dendrotoxin (DTX) block of K+ channel currents in Vicia stomatal guard cells was examined. Currents carried by inward- and outward-rectifying K+ channels were determined under voltage clamp in intact guard cells, and block was characterized as a function of DTX and external K+ (K+o) concentrations. Added to the bath, 0.1-30 nM DTX blocked the inward-rectifying K+ current (IK,in), but was ineffective in blocking current through the outward-rectifying K+ channels (IK,out) even at concentrations of 30 nM. DTX block was independent of clamp voltage and had no significant effect on the voltage-dependent kinetics for IK,in, neither altering its activation at voltages negative of -120 mV nor its deactivation at more positive voltages. No evidence was found for a use dependence to DTX action. Block of IK,in followed a simple titration function with an apparent K1/2 for block of 2.2 nM in 3 mM K+o. However, DTX block was dependent on the external K+ concentration. Raising K+o from 3 to 30 mM slowed block and resulted in a 60-70% reduction in its efficacy (apparent Ki = 10 mM in 10 nM DTX). The effect of K+ in protecting IK,in was competitive with DTX and specific for permeant cations. A joint analysis of IK,in block with DTX and K+ concentration was consistent with a single class of binding sites with a Kd for DTX of 240 pM. A Kd of 410 microM for extracellular K+ was also indicated. These results complement previous studies implicating a binding site requiring extracellular K+ (K1/2 approximately 1 mM) for IK,in activation; they parallel features of K+ channel block by DTX and related peptide toxins in many animal cells, demonstrating the sensitivity of plant plasma membrane K+ channels to nanomolar toxin concentrations under physiological conditions; the data also highlight one main difference: in the guard cells, DTX action appears specific to the K+ inward rectifier.