The experimental work performed in 6 dogs was aimed at studying the dynamics of morphological alterations of the interatrial septum (IAS) tissues in the place of occlusion of the endovascular appliance ("ASO") and around it on the right and left atrium side at different terms after operation. A defect of the animals' interatrial septum in the area of the oval fossa was modeled artificially and then abolished with the "ASO". No substantial differences were revealed between processes of organization of thrombotic masses and formation of the endothelium on the "ASO" from both atria. The endothelium on the micropreparations on the "ASO" surfaces was formed as early as by the 14th day after its implantation in the heart IAS.