Oxidative stress plays a pivotal role in the pathogenesis of many heart diseases. Patients with diabetes mellitus for instance exhibit an increased generation of reactive oxygen species (ROS). Approximately 25 % of the diabetics develop a pathological prolongation of the cardiac action potential (AP) the so-called long QT syndrome (LQTS). This AP prolongation can be attributed to an inhibition of the repolarizing potassium current IKr. The hERG potassium channel was identified as the main component of IKr. Both IKr in cardiac myocytes of rabbit and heterologous hERG currents in human cells are inhibited by endogenous ROS. Such a ROS induced inhibition of hERG could contribute to the AP prolongation of diabetics and to there higher mortality rates. Therefore hERG is an important target for the treatment of cardiomyopathies. The aim of this study was to identify the molecular basis for the ROS induced modulation of hERG. We focused on the modification of methionines (Met) und cysteines (Cys), because these residues are most susceptible to redox regulation. The main effect of Chloramin T as a Met- and Cys-specific substance and Sodium (2-sulfonatethyl)-methanthiosulfonat as a Cys- specific substance is a reduction of the current amplitude by up to 80%. This reduction of the current amplitude was observed for heterologously expressed and endogenous hERG channels. We show in this work for the first time that the ROS induced reduction of hERG current is mediated via specific amino acids and/or structures of the channel protein. We found that a ROS modulation of hERG occurs predominantly at cysteine residues, especially at Cys 723 in the C-linker. This Cys confers responsiveness of hERG to ROS and is located such it can interact directly with structural elements of the gating machinery and/or accessory proteins.