Abstract Two cases are presented which show presumptive evidence that the mechanism producing the Wolff-Parkinson-White syndrome may lie dormant for considerable periods of time. The observations reported may be of importance in ultimately explaining this phenomenon. ∗ ∗The observations reported here have been offered in support of the accessory pathway hypothesis. They contribute nothing to the irritable focus theory, either to confirm or disprove it, since irritable foci are notably temperamental in performance.