The aim was to investigate the effects of acute ischaemia on cardiac repolarizing K⁺ currents. Methods: We developed a model of acute ischaemia in isolated rat ventricular myocytes transiently surrounded with a mineral oil droplet. During ischaemic challenges, we recorded intracellular pH using the fluorescent probe seminaphthorhodafluor-1 (SNARF-1) and whole-cell K⁺ currents using the patch-clamp technique. Results: Decrease in intracellular pH (pH_i) during simulated ischaemia was dependent upon the extracellular proton buffer used (pH_i decreased from 7.44 ± 0.02 to 7.16 ± 0.04 in a Hepes-buffered medium and from 7.08 ± 0.04 to 6.56 ± 0.07 with bicarbonate buffer). In Hepes, action potential duration initially lengthened and then shortened under the effects of ischaemia. Initial action potential duration lengthening was concomitant with a block of the inward rectifier K⁺ current, whereas late shortening corresponded with the activation of the ATP-sensitive K⁺ current. Similar changes occurred in bicarbonate buffer although with different amplitudes and kinetics. Patch-clamp experiments also showed inhibition of the transient outward K⁺ current. Brief transient episodes of ischaemia activated ATP-sensitive K⁺ current in only 20% of control cells (n = 21) but in 100% of cells treated with 15 μM cromakalim (n = 9). Conclusions: (i) Simulated ischaemia produces complex effects on repolarizing K⁺ currents including both inhibition and activation; (ii) cromakalim accelerates activation of ATP-sensitive K⁺ current during simulated ischaemia.