The anatomy of the antero-septal commissure of the tricuspid valve and its relationship to the membranous part of the septum are described in 72 normal hearts from subjects ranging in age from 25 weeks gestation to 13 months. Three major patterns were found. According to whether the contiguous leaflets were fused at the commissure, joined at the circumference of the orifice, or left a gap at the commissural site, the varieties are described as undivided, annular, or gap commissure, respectively. Commissures of annular type showed further morphological varieties, as did the patterns of division of the membranous septum into its atrioventricular and interventricular components. Dominance of the atrioventricular component and/or absence of the interventricular membranous septum were encountered in 38 cases out of 72 (52%). This finding demonstrates that dominance of the atrioventricular membranous septum during fetal life and infancy is not universal. The variability noted in the morphology and position of the antero-septal commissure was not associated with other positional variations of the tricuspid valve.