A human genetic defect associated with ’long Q-T syndrome’, an abnormality of cardiac rhythm involving the repolarization of the action potential, was recently found to lie in the HERG gene, which codes for a potassium channel(1). The HERG K+ channel is unusual in that it seems to have the architectural plan of the depolarization-activated K+ channel family (six putative transmembrane segments), yet it exhibits rectification like that of the inward-rectifying K+ channels, a family with different molecular structure (two transmembrane segments)(2-4). We have studied HERG channels expressed in mammalian cells and find that this inward rectification arises from a rapid and voltage-dependent inactivation process that reduces conductance at positive voltages. The inactivation gating mechanism resembles that of C-type inactivation, often considered to be the ’slow inactivation’ mechanism of other K+ channels. The characteristics of this gating suggest a specific role for this channel in the normal suppression of arrhythmias.