Morphofunctional properties of autologous unconserved and allogenic conserved vein implants were compared in dogs. After 14 months, 69.2% of autologous grafts and only 20% of conserved allografts remained functional. Histological examination revealed damage to all layers of the venous wall, although endothelial desquamation is the major-function. of graft thrombosis. Histological manifestations of damage to allografts were more pronounced. which corresponds to the level of thrombosis. However, these changes are reversible, and if the passability of the graft is preserved during the early period after surgery, they have no effect on the graft in a remote period.