Repolarizing stimuli applied during the plateau phase of a cardiac action potential do not always cause the membrane to return to a resting state Instead, the repolarizing stimulus may be followed immediately by a subsequent action potential. The subsequent action potential occurs even though the repolarizing stimulus returns the transmembrane potential to its resting level. This study used the DiFrancesco-Noble membrane model in a patch geometry to evaluate the membrane currents that initiated the subsequent action potential. These initiating currents were found to be predominantly calcium currents.