The permeability of Ru(NH3)(6)(3+), Fe(CN)(6)(3-), and I- through voltage-gated alamethicin ion channels in a planar lipid membrane was investigated by amperometric measurements using a microelectrode located near the membrane. The redox current at the microelectrode initiated by application of a potential pulse to the BLM was quantitatively analyzed to obtain permselectivity of the channel. The permeability of Fe(CN)(6)(3-) was ca. 10 times smaller than that of Ru(NH3)(6)(3+) due to electrostatic repulsion, indicating that the channel gives a high barrier to the permeation of multicharged negative ions.