For several decades, the existence in some hearts of accessory conduction pathways other than the principal pathway (AV node and His bundle) has been postulated. With the development of clinical electrophysiology it seemed important to try and correlate anatomical findings with electrophysiological anomalies. The results of 10 years of research in this direction are summarized here. Ten patients, most of whom died of cardiac arrhythmia, previously had intracavitary electrophysiological studies. The anatomical study required approximately 15.000 histological sections for each of the 10 hearts. During these 10 years we gradually became convinced of the existence of 3 accessory pathways, viz. Kent bundles, atrio-His bundle tracts and Mahaim fibres. In addition, correlations between the anatomical hypotheses inferred from electrophysiology and the results of this anatomical study proved excellent in most cases.