Induction of immune tolerance to the transplanted organ is the ultimate goal of organ transplantation. If this can be achieved, the use of permanent immunosuppressive drugs is avoided, and consequently the risk of complications such as opportunistic infection is avoided. However, whether tolerance can be induced depends on the organ. The liver is the organ in which tolerance is most easily achieved. On the other hand, pancreas, small intestine, and skin, which are highly immunogenic organs, are known to be extremely difficult. Heart and kidney are somewhere in between. In the case of pancreas transplantation, autoimmune reactions must be controlled in addition to rejection. Basic experimental studies are essential for the induction of immune tolerance. These experiments often require the use of inbred small animals, especially rats, which are relatively easy to transplant. To date, we have attempted to induce immune tolerance using various rat transplantation models.