In a previous study we characterized the whole-cell current carried by TASK-1 potassium channels, in rat ventricular cardiomyocytes, I_TASK, (Putzke et al. Cardiovasc. Res. 2007; 75:59–68). These results suggest that in cardiomyocytes of rodents I_TASK makes a substantial contribution to the outward current flowing in the plateau range of potentials and that this current component can be inhibited via a1-adrenergic receptors. In our current work we have studied I_TASK in mouse ventricular cardiomyocytes of wild-type (wt) and TASK-1 knock-out mice using the patch-clamp technique. We characterized the TASK-1 current as a pH-, A293- and methoxamine-sensitive current. Deletion of the TASK-1 gene in mice abolished this current, but did not lead to major adaptive changes in the mRNA expression of other K_2P channels or in the biophysical and functional characteristics of transient outward (I_to) or inward rectifier (I_K1) currents. TASK-1 knock-out (KO) mice had a prolonged QT-interval and broadening of the QRS complex compared to wt littermates. In TASK-1-KO mice, ejection fraction and end diastolic volume were similar as in wt mice, indicating a lack of haemodynamic disturbances by the TASK-1 knock-out. Thus, the changes in the electrocardiogram of TASK-1 knock-out mice were most likely not a secondary consequence of a defect in pump function. These findings suggest that TASK-1 might represent another susceptibility gene for arrhythmias, including conduction disorders. Since the published data concerning cardiac TASK-1 expression in different species varies considerably we analyzed the relative expression of TASK-1 in the different regions of the human heart using quantitative PCR. Our experiments show that in humans TASK-1 is preferentially expressed in atria, auricles and the AV-node, while expression in the ventricles, the IV-septum and the apex of the heart is less pronounced. Using immunocytochemistry and patch-clamp experiments of human right auricles we characterized TASK-1 channel expression and quantified I_TASK in the human auricles. These data indicate that I_TASK is involved in shaping atrial action potentials of the human heart.