Mechanism of intermittent preexcitation in the Wolff-Parkinson-White syndrome. The concept of electronically mediated conduction across an inexcitable gap. A man with intermittent preexcitation in the Wolff-Parkinson-White syndrome is reported. Once a sinus impulse was blocked in the accessory pathway, the block (ie, loss of preexcitation) was continued for a while until a PP interval reached or exceeded a critical period inducing sudden reappearance of preexcitation. This critical period was considerably longer than the effective refractory period of the accessory pathway. These findings are explained by the use of the concept of electronically mediated conduction across an inexcitable gap in the accessory pathway. It seems that when loss of protection was maintained, the sinus impulse was blocked at the site proximal to the gap, but the impulse passing through the atrioventricular node always reached retrogradely the site distal to the gap, and therefore, the next sinus impulse was blocked again. 