The configuration of the upper myocardial border, considered by us to represent the arterial orifice level, has been studied microscopically and with the help of reconstruction techniques, in 7 human embryos ranging from 6 to 9.5 mm crown-rump length. In the 6 to 7 mm stage the arterial orifice level is not situated in one plane but has a curved configuration and reaches up to the origin of the 4th and 6th branchial arch arteries. As a consequence the septation, by the aortopulmonary septum, starts at orifice level. In the 9 to 9.5 mm stage the arterial orifice level has not only a curved but also a "twisted" configuration. This implies that 1) the pulmonary trunk is very short whereas the ascending aorta is rather long, 2) the pulmonary outlet is rather long whereas the aortic outlet is very short, and 3) the position of the aortopulmonary septum is such that it extends a long way proximal in the pulmonary outlet but hardly any distance in the aortic outlet. This means that the position of the orifices as well as the relative dimensions of both outlets are already similar to those in the fully developed heart.