SummaryThe allogeneic heart transplantation in the rat is an excellent model for the study of transplantation immunity, since the precise moment of rejection can be determined.This model was used to study the rôle of the liver in the reduction of the immunogenicity of certain antigens.If the venous return of the transplanted heart is realized in one group of animals through the vena porta, and in a second group of animals through the vena cava, rejection reaction manifests itself in the first group after 11–13 days, while this occurs after 6–7 days in the second group. Work is in progress to study the mechanisms responsible for this significant difference.