A 73-year-old man had a fast atrioventricular (AV) nodal pathway accidentally ablated 4 years before, while attempting to ablate a septally located concealed accessory pathway (AP). After initiation of treatment with beta-blockers, because of systemic arterial hypertension, the patient presented to the emergency room complaining of a markedly diminished exercise tolerance. The 12 lead ECG showed an interesting AV nodal Wenckebach sequence, interrupted by P waves retrogradely conducted through the AP. The mechanisms explaining the ECG are discussed.