The pars membranacea is the last portion of the septum formed in the fetus, is lo-cated between the left and right ventricles in front of the tricuspid valve, and between the left ventricle and the right atrium, behind the tricuspid valve. It lies in close proximity to the aortic and atrioventricular valves and is considered to be the weakest part of the ventricular septum. The aneurysms of the membranous portion usually measure 1 to 2 cm in all diameters and their walls are composed of fibrous tissue forming thick trabeculae when seen from the left ventricle. They can project into different portions of the right ventricle or right atrium.