Abstract: In anaesthetized rats the onset of arrhythmia induced by aconitine was shown to be delayed by iv injection of dimethyltrilobine iodide(DMT)1 mg/kg. In anaesthetized guinea pigs the onset of arrhythmia induced by ouabain could also be delayed by ip injection DMT 1.5 mg/kg. DMT 0.25 mg/kg injected ip was found to protect rabbits against chloroform-adrenaline induced arrhythmia. APD₅₀ of ventricular myocardial cells of guinea pigs in situ was shown to be prolonged by ip injection of DMT 1.5 mg/kg. In isolated rabbit atrial muscle the functional refractory period was found to be prolonged by DMT 3.0x 10^-6M.