Astragaloside IV (AS-IV) is one of the main active constituents of Astragalus membranaceus , which has various actions on the cardiovascular system. However, its electrophysiological mechanisms are not clear. In the present study, we investigated the effects of AS-IV on action potentials and membrane currents using the whole-cell patch clamp technique in isolated guinea-pig ventricular myocytes. AS-IV prolonged the action potential duration (APD) at all three tested concentrations. The peak effect was achieved with 1×10⁻⁶ m , at which concentration AS-IV significantly prolonged the APD at 95% repolarization from 313.1±38.9 to 785.3±83.7 ms. AS-IV at 1×10⁻⁶ m also enhanced the inward rectifier K⁺ currents ( I _K1) and inhibited the delayed rectifier K⁺ currents ( I _K). AS-IV (1×10⁻⁶ m ) strongly depressed the peak of voltage-dependent Ca²⁺ channel current ( I _CaL) from −607.3±37.5 to −321.1±38.3 pA. However, AS-IV was not found to affect the Na⁺ currents. Taken together, AS-IV prolonged APD of guinea-pig ventricular myocytes, which might be explained by its inhibition of I _K. AS-IV also influences Ca²⁺ signaling through suppressing I _CaL.