Intoxication with local anesthetics can induce lethal cardiac arrhythmias. These are thought to be a consequence of the interaction witch cardiac ion channels, in particular, Na⁺ and Ca²⁺ channels. It has been shown that local anesthetics, e.g. bupivacaine, also inhibit the rapid component of the delayed rectifying K⁺ currents (I_Kr, hERG channel). The role of this interaction in cardiomyocytes with normal or with impaired repolarization is currently unclear. We hypothesized that in cells that lack the second, slow component of the delayed rectifying K⁺ currents (I_Ks), the hERG channel interaction would become unmasked. We therefore investigated the effects of bupivacaine on cardiac action potentials (AP) in control myocytes and in myocytes after pharmacological blockade of I_Ks (LQT1) or I_Kr (LQT2). APs of cardiomyocytes isolated from the left ventricular free wall of guinea pigs were recorded at 37°C. LQT1 was induced by 10mM chromanol 293B and LQT2 by 1mM E4031. At 3mM, bupivacaine shortened APs by ~22ms (n=36, p<0.001). Although most of the cells (34/36) displayed AP shortening, in two myocytes an AP prolongation could be observed. In LQT1 myocytes, bupivacaine shortened the AP duration only by ~11ms (n=35, p<0.05) and induced AP prolongations at a significantly higher rate than in control myocytes (8/35 myocytes). In LQT2 myocytes, a mean AP shortening of ~23ms (n=20, p<0.001) and no case of AP prolongation could be observed after application of bupivacaine. In summary, bupivacaine induces AP shortening in control myocytes underlining its prevailing interaction with depolarizing currents. The occasional occurrence of AP prolongation in control myocytes is well compatible with the ability of bupivacaine to also block hERG channels. This interaction may become functionally more relevant under conditions of impaired I_Ks (LQT1) but not of impaired I_Kr (LQT2).