Exposure to carbon monoxide (CO), a well-known toxin, results in cardiovascular complications, including arrhythmias¹.  CO blocks Ca²⁺, Na, and K⁺ currents and can induce early afterdepolarisation arrhythmias (EADs) in rat or guinea pig ventricular myocytes^2,3. However, the dominant effect of CO on ionic currents is species dependent and the proarrhythmic mechanism in man is currently uncertain. Here we examined the effect of CO on iCell² human induced pluripotent stem cell derived cardiomyocytes (hiPSC) cardiomyocytes, a model commonly used for human cardiac safety assessment. Whole cell current clamp recordings of action potentials (APs) were assessed using the CO-releasing molecule (CORM-2; 10 µM) or the inactive control (iCORM; 10 µM). Data are presented as mean ± SEM and the significance level was determined by Student’s t-test. Spontaneously beating cardiomyocytes (n=32) exhibited heterogeneous APs. These were grouped into atrial (34.3%), nodal (12.5%), or ventricular shaped APs (53.1%). The CO effect was examined on a ventricular shaped APs triggered with 5-ms depolarizing current injections at 1 Hz pacing rate. AP duration measured at 90% of repolarization (APD_0.9) was increased from 307.3 ± 34.1 to 423.4 ± 39.7 ms (n=6, p <0.05). The lengthening of APD₉₀ was mirrored by a decrease in the peak of the action potentials from 55.7 ±7.7 to 46.4 ± 7.4 mV (p <0.05). In some cells, a secondary rising phase, consistent with EADs was apparent. During iCORM perfusion, APD_0.9 was stable, with coefficient of variation (CV) of 4.8% and showed no significant change when compared to control conditions (CV= 6.7%). Examining the CO effect on spontaneous APs, exposure to CORM-2 resulted in a progressive prolongation of APs and at longer exposure (6 minutes), cells exhibited a slow depolarisation above baseline, followed by failure to repolarise. In guinea pig myocytes³, CO prolongs the AP due to inhibition of the rapid delayed-rectifier K⁺ current (IKr). In iCell² cardiomyocytes, IKr inhibition with E4031 (0.1 µM) prolonged the AP and at higher concentrations (1µM) induced EADs. These data show that CO has proarrhythmic effects on hiPSC cardiomyocytes and that inhibition of IKr has qualitatively similar effects. Further work is required to establish the direct effects of CO on IKr in iCell2 cardiomyocytes.