MinK is a widely expressed protein of relative molecular mass ∼15K that forms potassium channels by aggregation with other membrane proteins^,,. MinK governs ion channel activation, regulation by second messengers,, and the function and structure of the ion conduction pathway,. Association of minK with a channel protein known as KvLQT1 produces a voltage-gated outward K⁺ current (I_sK) resembling the slow cardiac repolarization current (I_Ks),. HERG, a human homologue of the ether-a-go-go gene of the fruitfly Drosophila melanogaster, encodes a protein that produces the rapidly activating cardiac delayed rectifier (I_Kr),. These two potassium currents, I_Ks and I_Kr, provide the principal repolarizing currents in cardiac myocytes for the termination of action potentials,. Although heterologously expressed HERG channels are largely indistinguishable from native cardiac I_Kr, a role for minK in this current is suggested by the diminished I_Kr in an atrial tumour line subjected to minK antisense suppression. Here we show that HERG and minK form a stable complex, and that this heteromultimerization regulates I_Kr activity. MinK, through the formation of heteromeric channel complexes, is thus central to the control of the heart rate and rhythm.