An opportunity to study the mechanism in pre-excitation was provided by 3 patients with unusual features: coexistence of right bundle-branch block and pre-excitation, occurrence of 2 types of anomalous ventricular complex, and simultaneous occurrence of pre-excitation and varying grades of atrioventricular block (first degree to complete). The conclusion was reached that a functioning structural atrioventricular neuromuscular bypass is present in these cases.