VERBEET, T.W., ET AL.: Perioperative Mapping of Parahisian Accessory Pathways. In 1989, two patients were operated for deep septal “parahisian” pathways in our institution. Three different mapping techniques were used. (1) Epicardial activation mapping with a belt of 21 bipolar electrodes positioned around the heart. This belt was positioned either on the atrial or on the ventricular side of the atrio-ventricular annulus in order to localize both the atrial and the ventricular insertion of the bypass tract. (2) Right intra-atrial activation mapping on the normothermic beating heart with a bipolar hand-held probe. (3) Right intra-atrial cryomapping at 0°C. The “parahisian” pathways are remote from the epicardium and the pattern of epicardial activation is different from that of the free-wall pathways. Case 1: The electrophysiological study showed a concealed anteroseptal bypass tract. The peroperative atrial epicardial mapping during orthodromic tachycardia (OT) showed simultaneous activation of the posteroseptal area and of the basis of the right appendage. Right intra-atrial mapping during OT showed an anteroseptal “parahisian” pathway. Case 2: The ECG and electrophysiological study showed a right posterior pathway. The first site of epicardial ventricular activation during atrial stimulation was the right posterior area, 30 ms after the onset of the delta wave. The first site of epicardial atrial activation during OT was the posteroseptal area. The right intra-atrial mapping showed a posteroseptal “parahisian” bypass tract. This localization was confirmed with cryomapping. Conclusions: Some patterns of epicardial mapping may suggest the presence of a deep septal “parahisian” bypass tract: retrograde atrial activation at different sites (mimicking activation among multiple pathways); delay between the delta wave and the first epicardial electrogram. Right intra-atrial activation and cryomapping are useful to confirm the diagnosis.