The detailed structure of a ventricular septal defect was compared in 90 hearts with complete transposition (concordant atrioventricular and discordant ventriculoarterial connections) and in 102 hearts with concordant connections at both junctions; the latter group was selected to include only cases with the septums aligned in the normal way. The interventricular communications observed in 13% of the group with complete transposition, which, in our material, had no counterpart in the hearts with concordant segmental connections, were of special interest. These defects, completely surrounded by muscle, were positioned around the midline on the right side of the septum but always lay under or partially under the septal leaflet of the tricuspid valve. The medial papillary muscle group was always to the "left hand margin" of the defect as seen by the surgeon. Because these defects lay within the boundaries set by the septal leaflet of the tricuspid valve, they would conform to the criteria for classification as inlet muscular defects but could equally be described as central or subtricuspid. It is significant that, in all those cases with histologic sectioning, the axis of atrioventricular conduction tissue ran to the surgeon's right hand margin. This position is markedly different from the pattern found in typical defects of the inlet septum, which are completely surrounded by muscle and extend to the posterior wall of the heart. In this more common situation, the conduction axis runs above the left hand margin of the defect. This finding has obvious implications for surgical treatment.