Various cardio-active stimuli, including endothelin-1 (ET-1), exhibit potent arrhythmogenicity, but the underlying cellular mechanisms of their actions are largely unclear. We used isolated rat atrial myocytes and related changes in their subcellular Ca2+ signalling to the ability of various stimuli to induce diastolic, premature extra Ca2+transients (ECTs). For this, we recorded global and spatially resolved Ca2+ signals in indo-1- and fluo-4-loaded atrial myocytes during electrical pacing. ET-1 exhibited a higher arrhythmogenicity (arrhythmogenic index; ratio of number of ECTs over fold-increase in Ca2+ response, 8.60; n = 8 cells) when compared with concentrations of cardiac glycosides (arrhythmogenic index, 4.10; n = 8 cells) or the β-adrenergic agonist isoproterenol (arrhythmogenic index, 0.11; n = 6 cells) that gave similar increases in the global Ca2+ responses. Seventy-five percent of the ET-1-induced arrhythmogenic premature action potentials, while for digoxin this proportion was 25 %. The β-adrenergic agonist failed to elicit a significant number of ECTs. Direct activation of inositol 1,4,5-trisphosphate (InsP3) receptors with a membrane-permeable InsP3 ester (InsP3 BM) mimicked the effect of ET-1 (arrhythmogenic index, 14.70; n = 6 cells). Inhibition of InsP3 receptors using 2 μM 2-aminoethoxydiphenyl borate, which did not display any effects on Ca2+signalling under control conditions, specifically suppressed the arrhythmogenic action of ET-1 and InsP3 BM. Immunocytochemistry indicated a co-localisation of peripheral, junctional ryanodine receptors with InsP3Rs. Thus, the pronounced arrhythmogenic potency of ET-1 is due to the spatially specific recruitment of Ca2+ sparks by subsarcolemmal InsP3Rs. Summation of such sparks efficiently generates delayed afterdepolarisations that trigger premature action potentials. We conclude that the particular spatial profile of cellular Ca2+ signals is a major, previously unrecognised, determinant for arrhythmogenic potency and that the InsP3 signalling cassette might therefore be a promising new target for understanding and managing atrial arrhythmia.