Viscum Coloratum Flavonoid (VCF) is one of the dihydro-flavonoids isolated and purified from the stem and leaf of Viscum Coloratum. It was proved both experimentally and clinically to have the effects of anti-tachyarrhythmias. The mechanism of this effect, however, was unclear. In this study, the effects of the drug on the fast response action potentials (FAP) in canine Purkinje fibers and guinea pig ventricular myocardial cells were investigated by glass-microelectrode technique. The results indicated that 0.1 mg/ml VCF accelerated the repolarizations of 2nd and 3rd phase of FAP, shortened action potential duration (APD), and slightly shortened effective refractory period (ERP) (canine) or unchanged (guinea pig). delta ERP/delta APD ratio was increased, therefore, ERP was relatively prolonged. All effects appeared after using the drug for 2 minutes, reached stable status after 10 minutes, and disappeared after washing off the drug for 15 minutes. It was suggested that VCF was the drug which exerted effects rapidly, maintained effects shortly, and had effects reversibly. The cellular electrophysiologic mechanisms of VCF on anti-tachyarrhythmias should be attributed to prolong the ERP relatively and abolish the reentry. This drug is particularly applicable to the tachyarrhythmias due to reentry.