We investigated the effects of a hot-water extract of Artemisia iwayomogi, a plant belonging to family Compositae, on cardiac ventricular delayed rectifier K＋ current (IK) using the patch clamp technique. The carbohydrate fraction AIP1 dose-dependently increased the heart rate with an apparent EC50 value of 56.1±5.5Ռg/ml. Application of AIP1 reduced the action potential duration (APD) in concentration-dependent fashion by activating IK without significantly altering the resting membrane potential (IC50 value of APD50: 54.80±2.24, IC50 value of APD90: 57.45±3.47Ռg/ml). Based on the results, all experiments were performed with 50Ռg/ml of AIP1. Pre-treatment with the rapidly activating delayed rectifier K＋ current (IKr) inhibitor, E-4031 prolonged APD. However, additional application of AIP1 did not reduce APD. The inhibition of slowly activating delayed rectifier K＋ current (IKs) by chromanol 293B did not change the effect of AIP1. AIP1 did not significantly affect coronary arterial tone or ion channels, even at the highest concentration of AIP1. In summary, AIP1 reduces APD by activating IKr but not IKs. These results suggest that the natural product AIP1 may provide an adjunctive therapy of long QT syndrome.