SUMMARY The effects of intravenous procainamide infusion of 10-14 mg/kg body weight (i.e., 750 mg) of procainamide (PA) on reentry within the His-Purkinje system (HPS) were studied in 13 patients using His bundle electrograms and ventricular extrastimulus method. PA abolished reentry in eight patients (group 1) and decreased the width of reentry zone in the remaining five (group 2). At comparable S,S2 intervals, the S2H, intervals after PA were longer than control in all patients. In group 1 patients, after PA, reentry did not occur even at S2WH intervals that were significantly longer than control critical S2H2 intervals. In two of eight patients in group 1, PA abolished reentry by converting unidirectional block into bidirectional block in the antegrade limb (right bundle) of the reentry circuit. In the remaining six patients reentry was abolished because of consistent retrograde block of 82 impulse at some point between the site of stimulation and the His bundle recording site. In group 2, reentry was initiated after PA at approximately the same S,S% intervals as in control, but required significantly longer S2H2 intervals; in these patients the zone of reentry was shortened due to increase in effective refractory period of the ventricular muscle. PA significantly increased the functional refractory period of HPS and the effective refractory period of ventricular muscle. The results of this study differ from the previously reported effects of lower concentrations of PA which facilitated reentry within the same circuit. We conclude that the effects of PA on reentry are dose-related and can both facilitate and suppress reentry, depending on critical changes in conduction and refractoriness of the HPS.