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In 35 unselected patients premature right ventricular stimulation during a constant right ventricular drive rhythm was performed at the rates of 80, 100, and 120 per min. Several surface ECG leads and intracardiac electrograms were simultaneously recorded. To assess the beginning of decreased conductivity of the His-Purkinje system for retrograde conduction the premature test pulse interval was measured when the retrograde intraventricular conduction to the bundle of His prolonged for the first time. This interval was defined as the beginning of relative refractoriness of the Purkinje system. Additionally, the effective refractory period of the right ventricular muscle was determined. A rate dependancy between the beginning of relative refractoriness of the Purkinje system and the effective refractory period of the right ventricular muscle, as it is known from animal experiments, could be established with significantly different (P less than 0.001) mean values at the three different heart rates. The method described may be the only means by which to obtain approximate information about normal and abnormal conductivity of the human Purkinje system. It is inferred that critical prolongation of refractoriness of the Purkinje system relative to the refractoriness of the myocardium may be relevant to the occurrence of ventricular arrhythmias.