An 8 year old child suffering from asthenia and complete atrioventricular block, probably congenital. During the tread-mill test there was a sudden electrocardiogram change to a ventricular preexcitation pattern. Besides this rare association, we emphasize the importance of the autonomic balance over the accessory pathway's electrophysiological properties. The appearance of a latent accessory pathway might be a substitutive mechanism to counterbalance the serious impairment of physiologic atrioventricular conduction