In an effort to understand the molecular mechanisms of voltage activation of ion channels, we have chosen a system of known structure and examined the properties of heterodimeric channels formed between [Val1]gramicidin A ([Val1]gA) and [F6Val1]gramicidin A ([F6Val1]gA). Gramicidin channels are usually not voltage-dependent; but the introduction of a single symmetry-breaking dipolar F6Val1 residue into a ([Val1]gA)2 dimer to form the [F6Val1]gA/[Val1]gA heterodimer induces voltage-dependent transitions between two conducting states: a high-conductance state and a zero conductance (closed) state. The distribution between these states varies as a function of the applied potential but is not dependent on the nature of the permeant ion (H+ or Cs+). The permeating ions do not seem to contribute to the apparent gating charge.