The variability in the topographical anatomy of the conduction bundle at the site of the atrioventricular junction has been studied in four normal human hearts. The junctional area has been removed en bloc and serially sectioned. The conduction bundle and adjacent structures such as the posterior limb of the trabecula septomarginalis and the membranous septum have been reconstructed based on calculations from the histological sections. The study reveals marked variability particularly in the extent of the posterior limb of the trabecula septomarginalis. In one instance, the muscle was almost totally absent so that the branching bundle was located in a midline position and subendocardial both to the right and the left ventricular septal surface. In two hearts, the posterior limb of the trabecula septomarginalis had ramified so that only a small segment of the conduction bundle was covered by muscle. In the remaining case, a well-developed posterior limb of the trabecula septomarginalis completely covered the conduction axis thus accounting for the left-sided position of the bundle. The variability encountered may render the conduction bundle vulnerable to the tensile strain of the tricuspid valve apparatus, enhancing the natural process of wear and tear which may lead to disruption of conduction fibres and heart block, particularly in the elderly. Since the detailed topographical anatomy of the conduction bundle in the atrioventricular junctional area appears to be highly variable from one individual to another, meticulous inspection is mandatory once the area is manipulated at surgery.