Background: Hypokalemia reduces the cardiac repolarization reserve. This prolongs the QT-interval and increases the risk of ventricular arrhythmia; a risk that is exacerbated by administration of classical class 3 anti-arrhythmic agents. Small conductance Ca²⁺-activated K⁺-channels (K_Ca2) are a promising new atrial selective target for treatment of atrial fibrillation. Under physiological conditions K_Ca2 plays a minor role in ventricular repolarization. However, this might change under hypokalemia because of concomitant increases in ventriculay -60r intracellur Ca²⁺. Purpose: To study the effects of pharmacological K_Ca2 channel inhibition by the compounds AP14145, ICA, or AP30663 under hypokalemic conditions as compared to dofetilide and hypokalemia alone time-matched controls (TMC). Methods: The current at +10 mV was compared in HEK293 cells stably expressing K_Ca2.3 perfused first with normo- and then hypokalemic solutions (4 mM K⁺ and 2.5 mM K⁺, respectively). Guinea pig hearts were isolated and perfused with normokalemic (4 mM K⁺) Krebs-Henseleit solution, followed by perfusion with drug or vehicle control. The perfusion was then changed to hypokalemic solution (2.5 mM K⁺) in presence of drug. 30 animals were randomly assigned to 5 groups: ICA, AP14145, AP30663, dofetilide, or TMC. QT-interval, the interval from the peak to the end of the T wave (Tp–Te), ventricular effective refractory period (VERP), arrhythmia score, and ventricular fibrillation (VF) incidence were recorded. Results: Hypokalemia slightly increased K_Ca2.3 current compared to normokalemia. Application of K_Ca2 channel inhibitors and dofetilide prolonged the QT interval corrected for heart rate. Dofetilide, but none of the K_Ca2 channel inhibitors increased Tp–Te during hypokalemia. During hypokalemia 4/6 hearts in the TMC group developed VF (two spontaneously, two by S1S2 stimulation) whereas 5/6 hearts developed VF in the dofetilide group (two spontaneously, three by S1S2 stimulation). In comparison, 0/6, 1/6, and 1/6 hearts developed VF when treated with the K_Ca2 channel inhibitors AP30663, ICA, or AP14145, respectively. Conclusion: Hypokalemia was associated with an increased incidence of VF, an effect that also seen in the presence of dofetilide. In comparison, the structurally and functionally different K_Ca2 channel inhibitors, ICA, AP14145, and AP30663 protected the heart from hypokalemia induced VF. These results support that K_Ca2 inhibition may be associated with a better safety and tolerability profile than dofetilide.