Transcatheter closure of a membranous ventricular septal defect (MVSD) is much more difficult than closure of other intracardiac defects because of the proximity to the aortic and tricuspid valves and their relatively large size in small children. In this report, transcatheter closure of naturally occurring membranous VSDs was attempted in 12 Yucatan minipigs. The prosthesis is constructed from fine Nitinol wires in the shape of two buttons and a connecting waist filled with polyester fiber. Two kinds of prosthesis were used in this study: concentric and eccentric left-sided retention disks. A 6 or 7 Fr delivery sheath was advanced across the membranous VSD over a wire from femoral vein. The prosthesis was inserted through the sheath by pushing the delivery cable to deploy a button into left ventricle and the second button was then deployed into right ventricle by withdrawing the sheath. Successful implantation of the device was achieved in all animals except one. Complete closure rate was 58.3% immediately after placement, 100% at 1 week, 90.9% at 1 month and 3 months, and 100% at 6 months. An associated aneurysm of the membranous septum increased significantly in size in two of three animals using the concentric device, and in none of the animals using the eccentric device. A trace to mild aortic regurgitation was present in two of the three animals using the concentric device, and only in one of the eight animals using the eccentric device. Five animals developed a trace to mild tricuspid regurgitation. Pathologic examination showed all devices to be covered by smooth neoendothelium at 3 months. This report presents the first experimental study where closure of membranous ventricular septal defects in a swine model was attempted by specially constructed devices. Procedural success and occlusion rates are very encouraging but overall results cannot equal surgery. Further experimentation is needed with devices that are redesigned according to the experience gained from this study.