Electrical activity of the specialized conducting system (His bundle, bundle branches, false tendons and peripheral Purkinje fibers) of canine hearts has been recorded in situ through electrodes attached to the endocardium during total cardiopulmonary bypass. Simultaneous records from multiple sites have been correlated with electrocardiograms to indicate the sequence of activation of the specialized conducting system during normal and retrograde activation and during vagal stimulation. The results obtained indicate that the method employed has several advantages among which are an absence of injury at the recording site and an applicability to chronic studies.