Abstract: In anesthetized rats, arrhythmia induced by aconitine and BaCl₂ was shown to be antagonized by intravenously or orally administered puerarin, the main constituent of puerariae isoflavones. These two types of arrhythmia were also attenuated after oral administration of daidzein, the basic structure of puerariae isoflavones, and by equivalent doses of the alcoholic extract of puerariae which contains 40% of total isoflavones. Furthermore, ventricular fibrillation induced by CaCl₂ in rats and by chloroform in mice was prevented by daidzein or the alcohol extract of puerariae given orally. The duration of the ventricular fibrillation produced by occlusion of the coronary artery in rats was significantly decreased by daidzein given orally for 4 days. In anesthetized cats, the duration of monophasic action potentials and the effective refractory period of heart in situ were prolonged significantly by intravenous injection of puerarin. These results indicate that puerariae isoflavones can decrease myocardial excitability and may be beneficial in the treatment of myocardial ischemia.