The development of the outlet septum has been studied microscopically in 14 human embryos, ranging from 9 to 28 mm crown-rump length. Three tissue components are involved in the septation process: condensed mesenchyme of extracardiac origin, myocardium and endocardial cushion tissue. At the stage of 9 1/2 mm the condensed mesenchyme, which is embedded in the endocardial cushion tissue, is in contact with the myocardium at two sites. Graphic reconstructions of a 16 mm embryo show that at these sites of contact the myocardium is "drawn inwards" to form two bulges interconnected by the condensed mesenchyme. With further development the two myocardial bulges become the main mass of what then can be called the outlet septum. Thus the sites of contact between condensed mesenchyme and myocardium can be considered to represent the sites of attachment of the muscular outlet septum. This has important implications for the elucidation of the development of outlet malformations.