[Nodo-ventricular accessory pathway unmasked by a block in the trunk of the bundle of His].

A case is reported in which, contrary to the unusual finding, conduction was slower in an accessory pathway than in the node-His pathway. Only when this latter became blocked (in the trunk of the bundle of His) following on the longest refractory period, was a pre-excitation syndrome unmasked related to the right nodo-ventricular fibres (the fibres of Mahaim).