Endocavitary catheter ablation is a new method of treating cardiac arrhythmias. It may be used indirectly, interrupting conduction through the His bundle in supraventricular arrhythmias, or directly by altering the arrhythmogenic substrate to prevent ventricular tachycardia or fibrillation. We have observed, together with other workers, that changes in the electrical properties of the catheters used to deliver the endocavitary electrical discharge. This was unexpected as the shock was delivered through a unipolar electrode with the indifferent electrode placed on the patient's back. An analysis of the morphology of the discharge impulse in intensity and voltage at increasing energy values showed that the unconnected electrodes of the multi-electrode catheter were raised to approximately the same voltage as the indifferent electrode. The change in electrical properties of some catheters was therefore the result of an insulation defect between the conducting wires leading to the electrodes. A study of the rupturing voltage of 32 USCI endocavitary catheters showed that 5 were able to sustain repeated shocks of 2 Kv and that 3 were able to sustain 3.5 Kv. The insulation of Cordis catheters was good but problems were encountered due to the resistance of the wires leading to the electrodes which dissipated an abnormally high amount of energy, so reducing the energy actually reaching the myocardium. These observations may explain the inconsistency of results obtained by different groups performing endocavitary catheter ablation with material not designed specifically for this purpose.