Polychlorinated biphenyls (PCBs) have been known as serious persistent organic pollutants (POPs), causing developmental delays, motor dysfunction. We have investigated the effects of two PCB congeners, 3,3′,4,4′-tetrachlorobiphenyl (PCB 77) and 3,3′,4,4′,5-pentachlorobiphenyl (PCB 126) on ECG, action potential, and the rapidly activating delayed rectifier K+ current (IKr) of guinea pigs’ hearts, and hERG K+ current expressed in Xenopus oocytes. PCB 126 shortened the corrected QT interval (QTc) of ECG and decreased the action potential duration at 90% (APD90), and 50% of repolarization (APD50) (P < 0.05) without changing the action potential duration at 20% (APD20). PCB 77 decreased APD20 (P < 0.05) without affecting QTc, APD90, and APD50. The PCB 126 increased the IKr in guinea-pig ventricular myocytes held at 36°C and hERG K+ current amplitude at the end of the voltage steps in voltage-dependent mode (P < 0.05), however, PCB 77 did not change the hERG K+ current amplitude. The PCB 77 increased the diastolic Ca2+ and decreased Ca2+ transient amplitude (P < 0.05), however PCB 126 did not change. The results suggest that PCB 126 shortened the QTc and decreased the APD90 possibly by increasing IKr, while PCB 77 decreased the APD20 possibly by other modulation related with intracellular Ca2+. The present data indicate that the environmental toxicants, PCBs, can acutely affect cardiac electrophysiology including ECG, action potential, intracellular Ca2+, and channel activity, resulting in toxic effects on the cardiac function in view of the possible accumulation of the PCBs in human body.