The activation of inward-rectifying K+ channels in guard cells at membrane potentials negative of the K+ equilibrium potential is important for their cellular function as proton pump-driven K+ uptake pathways during stomatal opening. In animal cells the voltage-dependence of inward-rectifying K+ channels is produced to a large extent by intracellular magnesium block. In guard cells, when cytosolic Mg2+ was either 3 mM or < 1 microM, activation times, deactivation times and the steady-state voltage-dependence of K+ channels remained unchanged. It is discussed that the activation mechanism of inward-rectifying K+ channels in guard cells is independent of intracellular Mg2+ block.